Since the ships start side by side and steer at the same time, making a 90 degree turn would result in one ship being in front of the other.
I think the same way as with 3-way split in CF2 (only here it could be 2-way split but with both ships facing forward), the ships should not go paralelly but a bit to both sides, which would mean you can turn 90 degrees after a short moment. Splitting to the sides and then going paralelly would pose problems with hitting curves when the ship(s) are being moved to the side, while both ships starting very close to each other (where your ship previously was) and moving a bit away from each other would solve it (at the expense of the ships being harder to control after some time, but this might be considered either in plus or in minus).
I think 3-way split was one of the more fun PUs in CF2, so I think reviving it - maybe even as two different modules, one (a bit lower class) working exactly like CF2 3-way split, and the other (a bit higher class) working like Wtfusername proposed, would be cool.
Both those ideas also would’ve been really different to all existing modules, which is also great - there would be some significantly different gameplay experience / strategies both by people using these modules and for people defending against them.
For balancing, in FFA killing or cutting either of two splitted ships should give you half the points (which is fair, you only killed half of a player - well, maybe +1.5 for cutoff is not ideal, but I agree with what a lot of players already said that after final balancing cutoff worth should be more close to direct kill, so maybe it’ll not a be problem if it’s an even number. Or just (cutoff+1)/2 points for a splitted ship, because cutting one of two ships is more dangerous since you need to come close to the second ship that can kill you) as otherwise people would hunt for easy points since obviously it’s way easier to shot down a ship of a player who needs to think about two ships at the same time.