VDC booster effect on LiFePo battery run time?
If you put a 12V -> 24V converter between your power distribution setup and the monitor, the 50W monitor will only draw 2 Amp from the converter, but the converter will draw 4 Amp from your 12 V source - the inverter needs to draw 50 watts (actually, a little more to allow for inefficiency) from the power source to be able to feed 50 watts to the monitor.

I suggest a separate battery for each monitor - that may give 5 hours run time (perhaps less, depending on how far you can safely discharge the battery) for each monitor.