Multiple ground planes with optocoupler
Your concerns are justified, as two adjacent ground planes are just perfect for capacitive coupling of high-frequency noise from one to the other.

The standard approach is called a split plane. A split plane is just what it sounds like: your ground plane is divided into two separate areas which are not connected except for one point. This keeps the bridge noise (on one ground plane) separate from the signal processing (on the other plane). The disadvantage with this approach is the need for isolated IO. Your power connections, for instance, have to come in on two separate connectors, one for each plane. Likewise with signal connections. And, of course, you need to restrict the circuits to the area above the appropriate ground plane.