Why do it the first time at a show? Just hook the stuff up in your own home, crank the stereo and record.
If you are running a passive splitter, then the input impedances of the two recorders will both load the mic signal. The same waveform will appear at the input to both recorders, but the waveform will have been loaded by the parallel combination of both input impedances. So, if your problem is related to loading of the signal, both will see the same thing and you won't know which of them loaded the signal or whether it was the combination of both of them. (I doubt that either will significantly load the signal and if it does it will be in the form of moving the bass rolloff corner frequency to a higher frequency.)