Hi peterpete,
From how you describe it, the unit below will still need to support part of the wall that is remaining just below 1st floor level and possibly support the floor depending which way the floor joists run. Therefore a beam may still be required for the bottom unit work, and a larger one than the upper one if it is supporting the floor. There could also be some fire rating work to the 1st floor by the bottom floor owner.
Therefore there might not be a great advantage to you by doing the work together, but it will depend on the full details being revealed. I do feel however that tying your work to another owner’s work will introduce a layer of complications that I think you don’t need. For me in that situation, I would prefer to push on and get it done my way and avoid all the issues that arise when having to deal with another player.
I dont think there is a mechanism at present that can force an owner to agree to supch changes, but I could be wrong.
I assume that you are aware that both owners would require Councill approval (for structural work) and OC approval (for work to Common Property).
Regards,
Rob T