From my previous post, I found out from the comments that there are various generalisations of AdS/CFT with different things replacing the CFT on the RHS; such as AdS/CMT, AdS/QCD, and also with the AdS replaced on the LHS, like Kerr/CFT a hydrodynamic dual, etc.

I am thus led to ask, "*Is there a generalisation of AdS/CFT with string theories on both sides?*"

I can think of at least 1 example of a/n (holographic?) equivalence between a $D$ - dimensional string theory and a $D+1$ - dimensional string theory, T-Duality. E.g. the Type I String Theory and the Type I' String Theory, etc.