A1000 or A500 Back plane

[AMC258]

Please explain.  I can't tell exactly what you are talking about here.  Do you mean you want to put a Minimig in an A1000, or you want to convert a Minimig to a full A1000, or you want an expansion bus for the Minimig???

[AMC258]

Put two two-layer boards together and you get a very thick three layer board, not a four layer board.  Three two-layer boards gives you a way too thick four layer board.

You could put a mylar sheet between them though and get what you are trying to get, along with some extraneous capacitance.