This question requires more factual background to answer. Is there are rental business? Do both own the business? Did the owner who signed have authorization from the co-owner? What type of legal ownership do they each possess? You should gather as much information as you can and take your lease and any correspondence (including emails, of course) to a local lawyer.
It would not be safe to assume that because both owners did not sign the lease that the lease is invalid. There are many arrangements that could exist between the two owners which would give only one of them the authority to execute documents that would bind both. Many more facts about the relationship between the two owners would be needed to come to the conclusion that the lease is invalid.