Skip the transfil.
Sounds like your dive profile is like I was thinking. Recreational profiles in sidemount gear to get comfortable for eventual tech diving.
Back to the simple. Dive one, use a single tank with the other as a huge pony. If someone has an OOA, you have a huge pony to suck off of.
Two of you both have an OOA, WTF is going on? You would be completely screwed in normal recreational setup. You are not watching your gas usage. Take a step back and get gas management under control first. By the time you are trying sidemount you should be aware enough of your surroundings this won't happen. One of the two of you might screw up and do an OOA (or system failure) but not everyone. There is always the old school of sharing a reg, alternate breaths. OK, I have gone down this stupid tangent too far.
2nd/3rd dives start with 2 fresh tanks and practice real sidemount diving.
End of the first dive you have one empty and one full tank. End of second dive you have 2 half filled tanks. End of third dive you have all tanks empty.
Never going into the water with only a single tank of air, you always have two. The only bad is some unbalanced on the end of the first dive with one full and one empty tank.