Most efficient would be to disconnect the apartment’s power and sit in the dark and sweat.
The OP asked how to cool BOTH rooms most efficiently… To not fight the hypothetical, I’ll assume that means the least power consumption overall.
Is it not true that the most efficient AC unit is one which runs continuously at fixed output to maintain the desired temperature, rather than an overpowered unit that pulses on and off? So theroretically, if a single wall unit in either room could keep both rooms adequately cooled, perhaps with the use of a mixing fan in a doorway, but it had to run 100% of the time to do so, that would be more efficient than both units running 50% of the time.
So first, see if one unit can cool both rooms to your satisfaction; ideally without a mixing fan since that will add heat, but with the fan if the temperature gradient is unacceptable. If that doesn’t work, then it sort of depends on how controllable the units are: single stage compressor? Dual stage? Variable speed? If variable, and still assuming that one unit alone is underpowered, it would be most efficient to run them both continuously at the lowest power level possible. For single stage compressors, it’s a balancing act: if one unit alone is not *quite *enough, then both together are too much. You could turn the thermostat on one unit way down so it ran continuously, and set the other one at your goal temperature and let it pulse five or ten or twenty or a hundred times a day (more is worse for efficiency, but the precise number is out of your control at this point). That would be more efficient than both units being set to the same temperature and both turning on and off more often. You could even move from room to room and tweak the thermostats manually to alternate the ‘colder’ unit to even out the wear and tear; you get the same number of pulses per day but divided evenly between units, at the expense of you monitoring things a lot more closely.