I can show you where you might start looking, but I don't have an exact answer. You can check
here . I would guess that HDMI2 would be one of the newer ones that are in either 151 or 119 device, most likely device 151, command 66 (Video 7), using Sony15 protocol.
If you have a JP1 remote, it's fairly easy to program this in. If you have Pronto, go to
this page on the above site and click on the "How to use these codes with a Pronto remote" link on the bottom to use these codes. I don't know how you use these codes on Harmony.