Well you could of course wear them together. Depending on which you want to wear on the outside, make sure it's sized properly so it fits over the other. The Acto seems like more of the outershell.... but if you treat the Atom LT like a down jacket and wear it on the outside I think you'd be warmer (because it has more loft). Anyhow, they could easily be worn together and make a very warm set up. Make sure you size the layering properly or the fit will be very constricting and tight.

think i need to order one in a larger size than the other?