First, without evidence of illegal activity it would be difficult to evict. However, if you notice the evidence out in the open while doing maintenance or a periodic property inspection that would probably be sufficient for eviction. You'll have to check with an attorney.
This could easily be one tenant not liking another tenant and trying to make a problem for the landlord. Personally I'd attempt to stay out of the middle of it. Mainly because I'm not law enforcement.
If you want to you can end one or both tenancy(s) when the lease expires. It doesn't matter whether one or the other has any other place to go. As landlords we are explicitly not in the business of managing other peoples lives, just our property.