Homemade UPS Backup, couple questions?

I am looking to build a custom UPS backup. I have like 4-6 old UPS' from 1500VA to 650VA but the batteries have died and we just bought new units. What I was thinking is if I purchased one large sealed deep cycle or gel battery and connect a couple of these to it, would it work? I am thinking not since the charger part of the ups will see the other chargers and either overload or just not work.

So the other thing I was thinking is using the largest unit I have and use that as the charger portion and just run inverters or leave the AC unplugged from the other UPS' for the load. I know this will tax the charger unit and may just be too much load to keep the battery charged since it will be acting like an "online" UPS with a constant load off the battery. Am I thinking correctly?

I know the best way would be one large inverter/charger but i thought maybe i could make something work with what i have. Or maybe this is just a dumb idea?

If you cant tell, I'm looking for the cheap option for power backup for my small server, network, home theater. Total load would be around 1500W-2000W.

http://dansdata.com/upsupgrade.htm
I suggest using deep cycle batteries as they cost about the same but last longer. When using multiple series batteries, make sure to use some of the same model and age (preferably from the same batch) so they'll be nicely balanced.

Just plug all of them in as there shouldn't be any issues having multiple chargers connected to the same battery bank, as long as the charge rate of the batteries aren't exceeded. (In practice, that's highly unlikely as deep cycle batteries can take a lot of charging current.)

Here's a cool read on a custom ups setup. Well worth the while.
http://www.butkus.org/ups/ups.htm