Do both gray tanks drain to the same dump connection? If so, get a twist on valve for the dump connection and open both gray tank valves so they equalize.
I do this sometimes when dry camping if one gray tank gets full and the other still has some space.
A valve like this...
http://www.amazon.com/Valterra-T58-T...=sr_1_1&sr=8-1
Regards,
Alan