Why couldn't you form a stepped pad? It would require some knowledge of concrete forming, but it's not too hard.
What I mean is you have a step all the way around the outside of the pad for the sill to sit on, and then it goes up inside of the sill, rather than pouring a pad, setting the frame, and pouring another pad. If you'd do that you would have to have rebar sticking out of the bottom pad to tie in the new pad, and that would be awful to work around
like this:
OR, alternately, just have the sills above floor level. If you have a concrete pad with radiant heat, there is no need for a framed floor at all. And with a few minor adjustments, the sill can be above the floor level with no real problem (which would save you a few thousand dollars of concrete)
