If your DA specifically and clearly says that there is one car space for each unit, the the by-law is invalid. It would require approval from the council to change the allocation.
If the exclusive use by-law is valid (and after two years the Act ‘assumes’ all required preconditions were met) then you are stuck with it.
You might, however, be able to challenge it on the grounds that the EC at the time unduly influenced the Owners Corp and effectively conducted a land grab.
At the very least, the occupants of the additional common property parking should have been paying rent to the Owners Corp.
That said, I can’t see this getting anywhere unless you talk to a strata lawyer.