Probably, but not for very long. The amount of wattage that it takes to charge up a good UPS, will surely be greater than what a second UPS would be able to put out. Everytime you charged them off of eachother you would get less and less power until eventually, you had so little, you wouldn't be able to charge the other UPS, then.. failure. If you want to go with two.. then i would say keep both charged.. and after you use the first one and it dies, just hook everything up to the second one. Also, if you charge batteries with less watts than what is actually needed to charge the whole thing, the batteries will get damaged. It's basically the same principal with computer batteries and ipod and cell phone batteries. The less time you charge it for, when it really need to be fully charged, the more damage you will be doing to the battery. When batteries lose their charge, they are meant to be fully charged, to work correctly again. If not, you will be lessening their strength more and more over time.
As im sure we all know that overtime, rechargeable batteries begin to SUCK.. well by doing something like what your asking, for long periods of time, you will just be shortening the batteries life expectancy a whole lot faster than what it's meant to be.