I'm not familiar with ASP 25's, but as with any other twin be sure the engines are reliable, especially in the mid range where most of the flying will be done. I used a pair of old HB'25's on a twin seaplane about the same size as the Sig sealane and it had plenty of power so your 25's should be fine. My current project is a Sea Cruiser (designed for a single 40/45) and I am using a pair of OS 26 four strokes. From my experience it should have lots of power.
Peter