Are the current owners of the two lots likely to stay there for the long term? If so, why not simply mutually agree to let the other lot use your parking space without anything formalised? If one owner were to sell, the remaining owner could then negotiate with the new owner of the other lot to see whether they would be happy to continue the mutually convenient arrangement? That would cost nothing.
If the parking spaces are common property but allocated to particular lots, then a general meeting resolution could swap the allocation. Since it would not affect anyone else, there should be no objections.
If the parking spaces are included in the title of the units, then a formalised swap should be possible via some bureaucracy, but is it worth the cost and bother when you could do the first option above?