I'm a bit puzzled here ... do you have to give the FMC both a TA and a TL? Can't it derive the TL from the TA and the current altimeter setting? (This assumes that the crew keeps the local QNH up to date somehow while in the flight levels; I don't know if that actually happens.)
FMCs are very complex and while the sim add-on I use is extraordinarily faithful to the real thing, it cannot simulate every detail of FMC operation, or at least the current version probably cannot. For example, my impression is that a real FMC can fly a smooth DME arc, but in simulation the arc is simply defined as a large number of closely spaced internal waypoints (but maybe a real FMC does that too).