I can not see how you can do this without a MIDI Merge box and that still may be an issue if all the MIDI

I have been feeling your frustration on this trying to set this up myself, without a MIDI OUT and THROUGH to merge I do not know a way of doing it other than setting the MIDI Channels to be the same, if you do this then you can set one of the Instant Access buttons on the Ground Control Pro to send MIDI CC for a value and then set the same CC Number on all the Factor Boxes for TapTempo…BUT this creates other problems and is not really practical for me and may or may not be for you.