My reading of the original post is that OP owns both units, so there is no issue of getting “agreements”. As long as that continues, I’d guess he is free to allocate the two spaces as he sees fit. I’m also assuming the OP actually lives in one, and has a tenant in the other.
If the car spaces form part of the lot, this should show on the Plan for the building and the lot title deeds. If one or both of the lots are subsequently sold, the status quo would need to be resumed unless the plans are rewritten. If the spaces are on separate title, the OP could sell off which ever one they preferred?