I have already made sure that before I spawn them, to make their vehicle the "Same vehicle as player" option. Despite this, my partner still goes into their own vehicle. Anybody know a fix?

Turns out, I was a dumbass.

I didn't realize putting it to "None" was the solution. I thought that by "Same vehicle as player" the game meant that you and your partner would share vehicles, but it only gave your partner an exact copy of your vehicle.