Only running 3 psi nominal kinda indicates most all the air is going through your 7' diffuser and very little to your 12' diffuser, plus your 300' run loss is tenths of a psi (good). To get to your 12' deep diffuser, it would take (62.4 lbs/cu ft * 12 ft / 144 sq in/sq ft)=5.2 psi. Then you have the diffuser loss to add in, so you'd be seeing 6 psi or so. Such a system would take the path of least resistance in the vernacular, and go out the shallow leg. You don't mention valves at your split to regulate flow. May want to add a simple ball valve on each leg and close down the shallow leg a little. Your goal might be to have equal flow thru each leg and that would require an additional pressure drop on the shallow side to equal the added depth on the deep leg.
