Why two drains for simply draining the tank? Sure, go for it, but I would plug one unless you need to drain that sucker really fast.
Put a gate valve on one of the overflow drains and run it at full syphon. Use the other as an emergency and have just a trickle running down it. It will keep things quiet and there's no way you'll max the single drain with two Eheims if the drain is running full syphon. I'd guess you'll have to dial it back about 1/3-1/2 regardless. Those Eheims will be pumping out close to 1500gph and a single 1.5" should handle around 3000 at full syphon.