If you want to play all 4 speakers simultaneously, run both amps independently in stereo and remove the GFS-3 speaker selector is the best approach.
By the way, your GFP555 has two sets of output - Main and Bypass. It is safe to connect one GFA555 to Main input and the other one to Bypass input, OR you can connect both GFA555 to either input via a splitter.
Hope this helps.