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

What Is a Double Salt? (with picture)