I am still confused as to what all this buys you that makes it more attractive than just dropping a second bizobj for the second combobox offers.
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>I think it's just me being me. I'm already looking at four business obects on this one form before the lookups, and five lookups, not just two. I broke the problem down as small as I could before asking about it - upsizing is easy. The app I'm writing is to replace a FPD2.5 app that was opening tables across a T1 line, so speed amount of data sent across the line are serious issues.

Ahh. Now we're getting to it. First of all, how big are the lookups? If they're small then this might not be a bottleneck to your bandwidth. If they are large (Several Hundreds of records each) then maybe delaying instantiation via the instructions in the MM Guide under Optimizing Applications - Dynamically adding controls' would help.
Usually lookups do not change often and are a pretty stable bunch of records. Once pulled down that should pretty much be it and so maybe initially downloading once and filtering for the specific data condition would be your answer.