Once you have decided what return pump you will use, most likely in the neighborhood of 1200 gph, use the link below to find your overflow size.
http://www.reefcentral.com/calc/drain.php
There is another calculator to find out head loss of your return pump too.
Are your skimmers both hang on type? If not I don't think 12" will be enough room for two skimmers and two heaters. IMO sell both of those and put the money toward 1 bigger skimmer.