Unless I am missing something in the picture, It looks like you are putting some significant side load on those gearboxes. You most likely will want to support your shaft with something other than the output shaft of the planetary gearboxes, such as a plate with a bearing in it that the shaft runs through.
From Andymark's Website:
"Mounting a large load to the end of an unsupported shaft may damage the gearmotor. The shafts on these motors are supported by two ball bearings making them fairly rugged; however, we do not recommend them to be used with large overhung loads. We recommend using a design that supports the end of the shaft. If it is not possible to support the end of the shaft, we recommend mounting the load as close to the face of the gearmotor as possible and cutting the shaft to length."
I speak from experience when I say: Nothing breaks planetary gearboxes faster than side load!
