I have a 1989 Four Winns 160 Freedom with a 90hp Johnson OB and I'm replacing the wood in the transom my question is when i go to double up the plywood should i put a layer of fiberglass and resin in between or just glue it then laminate it?

I was going to use 3/4 inch bc plywood i heard that will work as long as its fully sealed and its a quality grade.
That's fine, just precoat the wood with resin, before the installation. That prevents dry spots from the wood soaking up the resin.
I just wanted to add..that I think this is another case of using BC ply VS. marine ( If it was available ).
Less time .. less hassle .. It works out the same $$ wise compared to "making your own MG" .. just more time and effort to get to the same calibre of MG.
MG is a totally different level of wood. Just the warping of other cut plys and the Thirsty nature of them..prep time etc.. in my book is not worth the potential problems that comes with skimping on costs with ply....
YD.