How to hold co-signor responsible

[Sam Abe]

I have a property that was rented to a couple with a co-signor. The couple left but their daughther and son-in-law stayed back and are not paying rent now. 

If we file eviction the case would only include the daughter and her husband as all lawyers say we cant include the original tenants as they have left. 

What is the point of having a co-signor as now the eviction case wont involve any of the initial 3 lease signors. And eviction case cant even claim the lost rent as all lawyers say we have to file 2 seperate cases. One to evict and other to claim monetarty loss. 

Is this how eviction works?

Property in North Hollywood.