Hello RedLance,
I have 3 UPS connected to one large flooded battery pack. APC 3000w, 1997 area.
My UPS state it's ok to replace the battery while the UPS is on or off..
No problems to report. I assume the UPS loads are independent.
Forget the diodes and stuff. The UPS charges the battery based on voltage, diode will mess it up in many ways. Do not add.
When the highest voltage is reached they all stop charging. The three help speed charging, otherwise it would take days.
I have an external charger I use to do the final charge, I do this once per quarter. I keep the battery on line while I do this. Have not had any problems. Other than remembering when to run the finishing charge.
Possible suggestion, test the flooded battery with one UPS, Add the charger off. Discharge the Battery some, watch the UPS charge it. after it's charged turn on the finishing charger. See what happens:-) If nothing other than it gets charged further then add the other UPS to the line.
Have fun,
Scott.