The fact that both drain line meet to drain back down does limits your maximum flow, just think of the speed that is lost, changing direction for the flow slow's it down. If you give each drain it's own line to your sump will remove that problem, but will not make it 'magicaly' work as 2x2" -> 4" of drain. Going to 2 separate line will give you your maximum of 2x 1" 1/4, 2" 1/2 is better than 2", and that's without restrictions.
You dont show your main tank, but if the pipes for your overflow are also 1" 1/4, you could maybe change them to 1.5-2" and use a reducer to go through the 1" 1/4 for the bulkhead, this way it would create more head pressure at the level of the reducer (less air, more water). But it wont really change the generaly aggreed math of 1" = ~600Gal/H, you would just removing restrictions.
Hope that helps...