I see your problem. Swee Lee used to sell midi mergers, but that was a very long time ago. I've not seen midi mergers for a long time, only midi interfaces. But perhaps I've not been looking very hard.
Perhaps the best way to route them would be software. If you are using Logic, you should be able to create environments for the midi routing easily.
Another way about it is connecting your Axiom to your RD700 and using midi thru to connect to your Micro X. You'll need to do some programming to your RD700.
Software easier.
For the benefit of other readers, shel keys' problem is cannot be solved by a midi interface. The problem is that the Axiom and Micro X has only 1 midi in and 1 midi out each. He wants to control his Micro X with BOTH his RD700 and Axiom, and at the same time hook his Axiom to his Mac. He ran out of midi ports. At his current setup, both his Axiom and Micro X needs 2 midi ports each. That's why he is asking for a midi merger which merge midi signals together. Right shel keys?