Generally, failure to specify any proportion is taken to mean that the two parties have a ‘joint and several’ responsibility. That is “a claimant may pursue an obligation against any one party as if they were jointly liable and it becomes the responsibility of the defendants to sort out their respective proportions of liability and payment.”
The wording of the by law seems to also support that in the second part, where the two proprietors are lumped together as if they were one.
I would guess that if this ever had to be determined in some legal process, the division would be based on unit entitlements. It’s possible that UE corresponds to floor area, and that is the basis for the recalculation. But in absence of some such determination I believe that the Managing Agent cannot impose any particular proportion on either lot.