If the full rental is $650, then you simply ask the LL whose lease is in force. If he says the other one, you can move since yours would be invalid as stated earlier.

If yours is in force, then the LL needs to remove the trespassers since they are not your guests but former tenants the LL failed to remove when he rented the apartment to you. If he fails to do this, then you would have a right to void the lease but be cautious. If you left and he removed the other party quickly, you could get caught between two apatments.

I would push for a definate decision from the LL.