TL;DR unless Cover shafts Day 2 viewers, Start Again is pretty unlikely to be her unit song tomorrow
after OBP, ever since Splash Party all multi-stage concerts have always had the same number of songs. they did 17 songs today: 8 were solos, 3 were gen songs, 4 guest units, and 2 EN units
in comparison, tomorrow there is a max of 7 solos if FWMC get 2 solos (again...) and only Promise for a gen song. assuming all 4 guests sing again, that's 12 songs accounted for, so they need to have 5 EN units to equalize the setlists
there are three options:
1) everyone will get 1 unit song with the guests, and thus 1 EN unit each. today, they did 4 trios for the guests, leaving 6 members out (Mori, Kiara, Ina, Shiori, Raora, CC). that's not enough to do another 4 trios, so it's likely they would do 2 duets and 2 trios
that leaves us with the 8 members without an EN unit yet (IRyS, Kiara, Biboo, Nerissa, FWMC, ERB, CC, Raora) to fill the 5 remaining songs, which isn't enough members. we could have 4 EN units with that setup and be -1 song. in this scenario we can't have Start Again because Mori already did her EN unit
2) guest and EN units aren't distinct, everyone gets two units regardless of who's in them. in that scenario, we can have Start Again, but if that happens, four members use up a unit slot. that leaves us with only 10 slots left to fill 4 guest units + 4 more units
assuming all four guest units are duets, we have 6 slots left and can only make 3 EN units. this is also -1 song compared to yesterday, the difference is that now there are 7 duets and 1 quartet versus 6 duets and 2 trios
3) guests will double up tomorrow to form either trios or quartets. I find this the least likely scenario, as that would mean some members don't get any guest units, and the guest unit compositions are different between the days. at BD, it was Korone, Kobo +1, and Suisei, Moona +2 on both days
while this would reduce the number of unit slots spent on guest units, this also reduces the number of guest unit songs. we'd go down to 10 songs accounted for and need 7 EN units. even in the best case scenario of doubling up in trios, we'd only have 12 unit slots for 7 EN units, leaving us -1 song. and even worse, Start Again in this case would bring us down to 8 slots for 6 songs, which is -2 songs. OBP was also -2 songs, but it was 33 vs 35 songs. this would be 15 vs 17, a much larger difference
anyways we'll find out tomorrow. I hope it's Biboo