I currently have two five gallon undergravel filters that need to be connected to an air pump in order to work. For the setup I currently have, getting one pump and splitting it between the two filters would work better, but I'm not sure how big a pump I need. Anybody have any advice?

I once tried to power two sponge filters with one pump and was not unsuccessful. All the air tended to go to one filter or the other, and it was difficult getting the air pressure to go to both filters evenly. Best of luck.