I was trying to edit Battle #136 (the last fight on Disc 1) to include both Edea and Seifer but I've encountered 2 issues:
1) Seifer's Gunblade isn't synched to his animations; he'll attack but the sword stays in place + no SFX plays when physicals are used. The two actors aren't synched with each other.
2) The end of the battle causes a freeze after Edea's 'death quote' (unknown cause). A freeze also occurs if Seifer is in the 1st spot instead of Edea, with the game loading one of his text dialogues when Rinoa + Irvine show up.
In the formation editor Cactilo, I've been looking at the other formations where Seifer appears, including the one where both him and Edea are present at the end of Disc 2 and taking note of the flags used there. I also tried shuffling the order of the enemies in the formation but that caused other issues (Rinoa and Irvine are scripted to enter the battle at the start, I'm not sure where this is set from).
The three pics of their formation data:
I'm thinking it's either those unknown values in the bottom-right (not sure what they are, but the Gunblade ones seem to be consistent across every Seifer encounter except for the final Disc 2 one which uses a different 'actor') or it's some kind of hard-coded deal. I also see 2e5 used in the top two sometimes, or 209. Any info on how this works would be appreciated.
Edit: By chance, I used the wrong Seifer during a test and was surprised to see it working fine (used the Disc 2 version of him, where he fights the party alone). No text displayed when Rinoa & Irvine appeared but it otherwise seemed to be OK. I'm checking & comparing their AI at the mo.
Just to clarify, here's how it's going down now:
I've got the formation set up as 1) Seifer, 2) Gunblade, 3) Edea (using Edea's default flags and flags taken from Seifer #1 in Formation #195)
If I use Seifer #1 (Disc 1 Seifer) in that slot then only he will load, Gunblade + Edea won't be visible, and he himself will run his Death script as Rinoa + Irvine run in before the game hangs. But if I use Seifer #2 (Disc 2, Fight 1) then the fight loads correctly with gunblade synch, etc. albeit with the initial Rinoa/Irvine text missing (I think this might be run from Edea, but if she's not in the 1st spot of the formation it maybe doesn't play or something; don't know how the game triggers this as there's nothing in Edea's AI for it as far as I can see).