Experiment 1
Edit
Both Characters have their max potentials unlocked,
Waifu1 will fuse at level 9999 with a level 100 without increasing any Stat Up Value % until later.

Fused at lvl 9999 25 times with no stat % ups.
Fusion 0 | Fusion 1 | Fusion 2 | Fusion 3 | Fusion 4 | Fusion 5 | Fusion 6 | Fusion 7 | Fusion 8 | Fusion 9 | Fusion 10 | Fusion 11 | Fusion 12 | Fusion 13 | Fusion 14 | Fusion 15 | Fusion 16 | Fusion 17 | Fusion 18 | Fusion 19 | Fusion 20 | Fusion 21 | Fusion 22 | Fusion 23 | Fusion 24 | Fusion 25 | |
HP | 128335 | 807665 | 1050211 | 1329904 | 1522117 | 1570304 | 1703127 | 1812842 | 1939711 | 2099352 | 2213661 | 2311494 | 2406506 | 2508554 | 2520219 | 2625636 | 2590688 | 2772139 | 2835050 | 3149553 | 2862169 | 3055969 | 2977947 | 3254869 | 3150092 | 3058286 |
SP | 152754 | 840395 | 1227796 | 1451880 | 1480520 | 1754030 | 1847911 | 1959970 | 2099830 | 2351922 | 2352233 | 2542709 | 2578490 | 2694979 | 2692140 | 2910082 | 2989308 | 2975274 | 3196623 | 3219816 | 3254824 | 3338562 | 3379695 | 3569284 | 3360199 | 3570063 |
ATK | 31519 | 181615 | 560068 | 665220 | 735183 | 824824 | 862461 | 932066 | 997440 | 1050160 | 1155138 | 1260057 | 1233878 | 1313554 | 1323629 | 1386597 | 1420078 | 1433164 | 1538248 | 1575202 | 1582248 | 1577269 | 1520366 | 1718231 | 1666634 | 1748432 |
DEF | 37072 | 184883 | 550237 | 630230 | 720181 | 798114 | 861769 | 882759 | 973737 | 1018340 | 1119752 | 1168177 | 1233844 | 1260185 | 1314857 | 1398790 | 1394154 | 1394039 | 1470912 | 1478866 | 1549551 | 1586159 | 1611272 | 1643216 | 1610246 | 1710134 |
INT | 143203 | 400416 | 1095132 | 131875 | 1561364 | 1470371 | 1713436 | 1844718 | 1982601 | 1996987 | 2204872 | 2443110 | 2452858 | 2519914 | 2557065 | 4019574 | 2771921 | 2834682 | 2958042 | 3034347 | 3119605 | 3124201 | 3149878 | 3400844 | 3178765 | 3255350 |
MND | 104940 | 315157 | 973510 | 1083448 | 1260259 | 152771 | 1522515 | 1575030 | 1733301 | 1864180 | 1889868 | 1912853 | 2099801 | 2202245 | 2195622 | 2310051 | 2393982 | 2519761 | 2520061 | 2546109 | 2638870 | 2694765 | 1406665 | 2834981 | 2835411 | 2894062 |
AGI | 118813 | 209996 | 560076 | 653720 | 839724 | 840078 | 866017 | 963114 | 1015134 | 1050269 | 1135130 | 1244849 | 1260109 | 1329971 | 1340289 | 1372336 | 1469942 | 1406475 | 1470380 | 1680046 | 1586404 | 1575349 | 1575202 | 1650259 | 1819913 | 1757654 |
TEC | 105054 | 256686 | 720132 | 840127 | 936954 | 1041379 | 1155108 | 1233852 | 1260914 | 1360659 | 1449822 | 1545740 | 1574984 | 1594723 | 1712871 | 1758954 | 1889182 | 1826817 | 1889724 | 1905141 | 1898456 | 1006386 | 2030165 | 2030126 | 2077874 | 2192834 |
Total | 821690 | 3196813 | 6737162 | 6786404 | 9056302 | 8451871 | 10532344 | 11204351 | 12002668 | 12791869 | 13520476 | 14428989 | 14840470 | 15424125 | 15656692 | 17782020 | 16919255 | 17162351 | 17879040 | 18589080 | 18492127 | 17958660 | 17651190 | 20101810 | 19699134 | 20186815 |
Increase | 2375123 | 3540349 | 49242 | 2269898 | -604431 | 2080473 | 672007 | 798317 | 789201 | 728607 | 908513 | 411481 | 583655 | 232567 | 2125328 | -862765 | 243096 | 716689 | 710040 | -96953 | -533467 | -307470 | 2450620 | -402676 | 487681 | |
% increase | 389.05% | 210.75% | 100.73% | 133.45% | 93.33% | 124.62% | 106.38% | 107.13% | 106.58% | 105.70% | 106.72% | 102.85% | 103.93% | 101.51% | 113.57% | 95.15% | 101.44% | 104.18% | 103.97% | 99.48% | 97.12% | 98.29% | 113.88% | 98.00% | 102.48% | |
Rival Comparison | 54.02% | 34.22% | 21.46% | 19.57% | 12.84% | 11.75% | 9.56% | 8.11% | 7.19% | 6.39% | 5.85% | 5.10% | 4.59% | 4.21% | 4.28% | 3.57% | 3.36% | 3.19% | 3.06% | 2.79% | 2.71% | 2.55% | 2.55% | 2.43% | 2.38% |
Waifu2 will fuse how I normally will which is fusing with both members at level 9999 and making sure to get a 40% bonus each time.

Fused at lvl 9999 25 times with a 40% stat boost each time
Fusion 0 | Fusion 1 | Fusion 2 | Fusion 3 | Fusion 4 | Fusion 5 | Fusion 6 | Fusion 7 | Fusion 8 | Fusion 9 | Fusion 10 | Fusion 11 | Fusion 12 | Fusion 13 | Fusion 14 | Fusion 15 | Fusion 16 | Fusion 17 | Fusion 18 | Fusion 19 | Fusion 20 | Fusion 21 | Fusion 22 | Fusion 23 | Fusion 24 | Fusion 25 | |
HP | 128338 | 1340401 | 2855413 | 4289848 | 6518750 | 8924014 | 12106718 | 15628562 | 19422152 | 22878089 | 28543947 | 30869814 | 40493389 | 43669926 | 50410460 | 56289567 | 58586261 | 65611639 | 70025068 | 76866939 | 81649595 | 81505614 | 96225319 | 103282974 | 102671834 | 108403065 |
SP | 152751 | 1425973 | 2903336 | 4534012 | 6279825 | 9053433 | 11864983 | 16187576 | 21413003 | 24239377 | 26698420 | 32295117 | 36855044 | 45969514 | 50774591 | 52622016 | 61039778 | 66868216 | 73338445 | 77744309 | 84192119 | 91239374 | 94605796 | 102022803 | 99828620 | 107955183 |
ATK | 31517 | 419972 | 2139844 | 3611726 | 5144316 | 7698932 | 10320730 | 13322915 | 17279593 | 19821560 | 25864534 | 29514045 | 33965807 | 39294546 | 44640290 | 49927095 | 58716626 | 63008844 | 69254914 | 79454276 | 78350212 | 87333776 | 89435524 | 98063132 | 96100581 | 98799372 |
DEF | 37070 | 440931 | 1891108 | 3547042 | 5240586 | 7454571 | 10591864 | 13508236 | 17007150 | 21332662 | 27287580 | 30874133 | 34973278 | 38845410 | 44919014 | 47444306 | 58951154 | 61005867 | 69786307 | 79341585 | 78942842 | 88229105 | 87276869 | 95098213 | 94536660 | 113572670 |
INT | 141701 | 689934 | 2772400 | 4200361 | 6493906 | 8984357 | 11785494 | 14279056 | 18665603 | 23632442 | 26064282 | 29618528 | 37231303 | 43876942 | 47618284 | 53321057 | 58983265 | 63132460 | 69682445 | 73388141 | 82069510 | 92206233 | 95879544 | 92404290 | 102671833 | 109276353 |
MND | 104965 | 609572 | 2643234 | 4188242 | 6231881 | 8120005 | 11617876 | 15038887 | 18400224 | 22159452 | 26522817 | 31675226 | 37426749 | 40731015 | 45363455 | 53870814 | 60601706 | 64650215 | 70230584 | 74885942 | 81469449 | 86954856 | 46067374 | 98894769 | 110652836 | 112512824 |
AGI | 119055 | 466564 | 2099905 | 3360769 | 5248587 | 7944184 | 10405935 | 14127848 | 17696794 | 21967216 | 25321106 | 29926993 | 33685715 | 41312331 | 45237568 | 51446733 | 59621518 | 62976902 | 66705834 | 69656160 | 76648507 | 92962950 | 91027384 | 102674074 | 103863080 | 94977503 |
TEC | 105019 | 524923 | 2380630 | 3888850 | 5110644 | 7646221 | 10917010 | 15115201 | 18106682 | 21798631 | 25399485 | 31702683 | 36523849 | 42227058 | 42515971 | 50903647 | 56895787 | 63923095 | 71105723 | 75920187 | 99999999 | 42000517 | 91218255 | 97368350 | 100977367 | 103977429 |
Total | 820416 | 5918270 | 19685870 | 31620850 | 46268495 | 65825717 | 89610610 | 117208281 | 147991201 | 177829429 | 211702171 | 246476539 | 291155134 | 335926742 | 371479633 | 415825235 | 473396095 | 511177238 | 560129320 | 607257539 | 663322233 | 662432425 | 691736065 | 789808605 | 811302811 | 849474399 |
Increase | 5097854 | 13767600 | 11934980 | 14647645 | 19557222 | 23784893 | 27597671 | 30782920 | 29838228 | 33872742 | 34774368 | 44678595 | 44771608 | 35552891 | 44345602 | 57570860 | 37781143 | 48952082 | 47128219 | 56064694 | -889808 | 29303640 | 98072540 | 21494206 | 38171588 | |
% increase | 721.37% | 332.63% | 160.63% | 146.32% | 142.27% | 136.13% | 130.80% | 126.26% | 120.16% | 119.05% | 116.43% | 118.13% | 115.38% | 110.58% | 111.94% | 113.84% | 107.98% | 109.58% | 108.41% | 109.23% | 99.87% | 104.42% | 114.18% | 102.72% | 104.70% | |
Rival Comparison | 185.13% | 292.20% | 465.94% | 510.90% | 778.83% | 850.81% | 1046.10% | 1232.99% | 1390.18% | 1565.79% | 1708.20% | 1961.90% | 2177.93% | 2372.66% | 2338.46% | 2797.97% | 2978.48% | 3132.88% | 3266.74% | 3587.05% | 3688.65% | 3918.92% | 3929.04% | 4118.47% | 4208.07% |
Findings:
Edit
Once in a while the stats get reduced temporarily.