Does the lease have a provision for guests? An attorney can review the lease in its entirety and advise you accordingly with respect to any remedies and proper notice.
I understand you do not want to evict one of the tenants but you have an obligation to provide quiet enjoyment to the other tenant. If one of them has guests that are disrupting the other tenant you need to advise that tenant that there are rules that have to be followed.
This is not intended to create an attorney client relationship and none is to be implied either. You must contact an attorney and present all facts before you can and should act on this response