Is a lease on a renatl home official if only one out of the two parties signed the lease?

It depends on what the lease says about whether both parties must sign.  If the lease makes each party jointly and severally liable for the lease, then the party who signed as tenant may be liable for the whole lease even if the other tenant did not sign.  If the lease says it is not effective until all parties sign, however, then you very likely can void the lease without the last signature.  However, this may have to be taken to court if the landlord resists.