While it doesn't sound like much, sizing the ACP brass in the .45 Colt die could leave it a bit of a force fit in the ACP chamber. Quel bummer!
Using the ACP die to resize the .45 Colt brass would mean excessive working of the mouth of the case as the bullet is seated. This results in split cases, way too soon. Quel bummer!
Some corners ain't worth cutting. The best way to save money on reloading dies is to find some good used stuff at a gun show. If the innards of the sizer-die isn't visibly scored, it'll work--particularly for the .45 Colt. I'd recommend carbide dies for the ACP, since it would most likely be used far more often.