even with one single overflow you can split the overflow into two sumps , youd need two pumps one for each sump , thing about having two sumps with just one overflow in your tank is if the overflow ever became blocked or clogged(heaven forbid) youll have twice the mess
honestly if you have that much liverock left over sell a few pounds and set up another tank or donate the rock to me for frags
cheers and goodluck