ALL I know about double slats is they are slats have two anions or cations. I want to be clear about it. There is nothing detail about it in my book or in the websites I searched. Someone please tell me about double slats.

There are lots of places to look.

https://en.wikipedia.org/wiki/Double_salt

Introduction To Coordination Chemistry, Double Salt - Transtutors (http://www.transtutors.com/chemistry-homework-help/co-ordination-chemistry/double-salt.aspx)

What Is a Double Salt? (with picture) (http://www.wisegeek.com/what-is-a-double-salt.htm)