Suggestions on board designing, ground plane and layers - PCB images
In this case, what you want to do is split the planes -- your 5V power grid/net and ground plane are exclusively in the digital logic section of the board, while the 12V power grid/net and ground plane are exclusively in the power section of the board.  This means that there is a copper-free "moat" between the two sections of the board -- the only things that span the moat are the optocouplers and the isolated 5V supply "brick" module.