Now that I think about it, Unless you have some fairly sophisticated timing, You would probably still have a measurable spike when the lights switch over. So, how about this: A battery charger, 3 or so deep cycling car batteries, and a car inverter. That ought to be able to scrub out any little spikes that might occur when the lamps switch. You might be able to use a computer UPS from a company like APC, although you would need to make sure it can provide 800 watts for a short period of time (in case the lamps timing happens to overlap and both lamps are running).
That would probably be the best solution for you, 3 bateries, a car inverter, and a battery charger shouldnt cost much more than $350, Plus it would be better for the invironment (ever think about the implications of disposing of or even transporting 60 lead acid batteries? doesnt sound fun to me