The answer is most likely yes, but it would be very hard to quantify. IME and IMO many UGF's are run with less flow than they should be. so adding a second airstone would increase flow and subsequently increase the effectiveness of the ugf. IT is not so much that the bacteria need the extra waterflow, but that there may be spots in the plate that don't flow at all due to the path of least resistance principal. Years ago I went to powerheads over airstones based on this. Powerheads deliver steadier more consitant flow (not to mention quieter) so to me they are more desireable. This is not to say that airstone can't do a good job, only that powerheads do better IMO. With all of this in mind, the more uplift you use the more effective the ugf should be to a point. Please also remember that the more flow you have in a standard UGF, the more mulm will collect in the gravel and the more you will need to vaccum.
Dave
