I think you could do it the way you say , but dont ask me how.... Ive done it that way in the past...I think ..who knows....
Are you aware of this setting in the picture , its helped me, but beware when you name stuff , I think you need to remove any spaces in your naming and replicate the name of your retopo objects and vox tree objects exactly.
Or maybe this is for separate UV maps..
Stusutcliffe has it correct, Names Correspondence for Baking. It does not need to have multiple uv sets but each retopo object layer must have a sculpt room layer with the same name. Example shown in picture.
3DC will internally explode (pull apart) your model and bake each section separately to avoid occlusion errors.
Select and run what is shown in the second picture to check your names. In a simple case not needed but with lots of layers it is good to run before baking. Make sure Names Correspondence for Baking is selected. Not shown in the picture.