| I think the over flow rating is usually a maximum. I would be a little worried that two Mag 5s would be too much. But you could certainly give it a try. Just be ready to pull the plug quick if the overflow isnt keeping up. Also make sure your overflow stays clean and well flowing if you are going ot push the limit of it.
I have a mag 7 going on my 90g through 2 90deg and Id say tis doing at least 400gph. | 
