So, just so I understand the question, we're trying to determine if it would be ok, for instance, to take a dive that was 20 minutes at 100' and 20 minutes at 50' and base our next dive as if it were a 40 minute dive to 75'?
Since the pressure effects of sea water are linear with depth (not sure if the resultant effects of nitrogen loading are correspondingly linear though) then shouldn't this averaging be valid?
I'm not that experienced so consider all of this pretty much an addition to the OP's question.