I usually make the inner frame, measure it, and build the outer frame to fit. I use 1/16 instead of 1/8 allowance on some things, to get the best look.
If you aren't going to be able to cut both frames, measure your inner molding width as if it were going to be a mat. Measure the BACK of it, not the front.
Multiply by two, add it to your frame size, add your allowance, and there you have it.
Mixing and matching molding samples for stacked frames can be a lot of fun, and create a unique look. Also, some wide, shallow moldings can be turned sideways to use as a liner for shadowbox moldings, instead of strips of matboard and fomecore. Turn them inside out, and use as a really fancy fillet.
It's a challenge to your spatial relations skills, but worth it for the creative results.