Although is may not be a bad idea i prob. woudnt stack them for a few reasons 1. Too much forskolin can cause stomach or gi discomfort, with Alphamax your provided 50mg of a 95% extract which people tolerate well, however by addingyou would be adding an undisclosed amount of Forskolin from a 20% extract, which more commonly causes GI discomfort. Also by combining the two you would be getting 9000iu of Vitamin D3, which im sure some could try and argue is good or safe, i cannot recommend that.
I would try one, then the other and see which you respond to better. With Alphamax every single ingredient used is proven and is fully dosed, you know exactly what your getting, it would be my first pick for sure.