I don't think it would be very practical. To even ask that question, I have to assume that each tank is too small to run a big Ultima by itself. I can see plumbing each small tank with uptake and return lines. The real problem is that Ultimas use a lot of water during purging. Each uptake line would need a valve to open/close to avoid a potential catastrophe during purging. There are people who run a big sump to filter multiple tanks. I think that would be a better way to go when wanting filtration for multiple tanks. Using one Ultima to filter multiple tanks sounds like a nightmare to me.
Believe me, I am not against Ultimas and purchased two Ultima II 4,000s for biological filtration on my big tank system which still isn't ready to be set up. With a 405 gallon sump and nearly 1800 gallon tank, the 2,000+ gallon water volume will work for purging the two Ultimas.