You need Keyclone.

Synergy and Multibox will both broadcast to the 2nd machine, but neither will broadcast to many clients on the 2nd machine. Whether or not you would need two keyclone keys (one per machine) or if you could broadcast with Multibox and have keyclone pick-up the key presses sent and the then rebroadcast is up for debate. But as Keyclone is only $10 each I'd suggest biting the bullet and getting two.

This no doubt could also be achieved with AHK, but with much greater complexity. Way I see it is if I could program a stand alone utility like Keyclone I'd have the skill to make AHK work the way I want... I don't.