Would you like to consider using SOLIDWORKS PDM for the version management?
If you want to manage the revision manually, it could be troublesome when you dealing with larger assembly.
For the case you explained, the drawing B and drawing A will have file reference to Assembly A. That should not be an issue .
Just a thought,
If the assembly is same why do you need a new drawing number, you can also add another sheet showing alternate position (and an extra config in 3D) or add note on second sheet like you have added on new drawing.
This way you always have to update only one drawing in case if you make changes to "model A".
And if you are expecting changes in future in one of the drawings (and in assembly as well) then it'll be tedious to separate out assemblies, so it's easy to make a copy initially to avoid the confusion later if there is a possibility of revision.