This is a tricky question because it is very fact specific. I would look at the language of your contracts with these tenants as to what you promised them for parking. If the tenant with 2 cars had notice of the parking spot being designated (although public property) for the other tenant, then he would have agreed by contract not to use it.
If you don't have facts like that, then it will be hard to enforce anything against them. Further, if you promised parking availability to the other tenant under contract and now cannot provide it, you may be breaching those terms of the lease agreement.
You should meet with an attorney who can review your contracts and let you know exactly what your best course of action is. I hope this helps answer your question.