A few ships have double slots. These are the only ones I can think of:
ISD I- Double offensive retrofit
Mc-80 Battle cruiser - Turbolaser
MC-80 Assault cruiser - defensive retrofit
Pelta with Phoenix home - officer
But with any of these, I have never seen a combination that really struck me as having amazing synergy. And at least most of the time, my points are spent better other places. Is there some combination that I am missing?