Hello. I believe I can assist you.
First, if your lease is for a fixed term and that fixed term has not yet expired, then your landlord cannot unilaterally change the terms of your lease.
However,I am assuming your written lease was originally for a set term but that original term expired, and since then you have stayed on in the apartment. At which point your lease became "month-to-month." If this is the case, your landlord can change the terms of your lease (e.g. increase rent) provided sufficient notice is given. In California, landlords must given 30 days advance written notice.
As for removing your access to this parking spot, the same rule would apply. If the landlord wants to unilaterally change your lease to take away the designated parking spot, they have to give you 30 days written notice. There are exceptions to this rule in rent control areas, but I don't believe Hawthorne is rent controlled.
As far as options, you can assert your rights as a tenant under the lease as it exists, i.e. you have an exclusive right to use the spot. you can advise the landlord that you will sue the landlord for breach of lease if he does not return sole use of that spot. However, this is a short terms solution. As I said before, once he gives you 30 days written notice of these changes (the increased rent and parking spot) your choice will effectively be to either agree, try to negotiate further with the manager or find another apartment.
Assuming the owner and manager are not the same, you can perhaps appeal to the landlord directly on the issue. Landlords generally like paying tenants who aren't troublemakers and don't like to risk losing those tenants to new tenants who could possibly be a lot worse. The landlord may overrule the manager. The manager may prefer to try and placate the other tenant with 2 cars at your expense, but the landlord may prefer to keep you content and paying regular rent.
You are not without some bargain power, if you try to negotiate with the landlord. If you were to leave, the next tenant would almost certainly demand exclusive use of the parking spot as a condition in the lease anyway.
Good luck to you!