Optimize signal return path with decoupling capacitors in a two layer board
You are correct in your understanding. The return current from any signal will want to follow the same path as the signal itself using an adjacent ground or power plane. If the ground plane is broken it will still find a path back to the source of the signal, but by a longer less optimal path which can result in higher emissions and worse immunity. Whether this is a problem in your design depends on many factors such as the clock speed of the signals, and more importantly the speed of their edges.

If you think it may be a problem (and presumably you do) then the best solution is to use a 4 or more layer board so you have an unbroken ground plane. Using a 2 layer board you could add a 0805 or 1206 zero-Ohm link to stitch the two ground planes together at the point where they are broken to provide the current return path.