I was having a hard time figuring this out too, until I finally found the instructions on pages 81 and 82 of the 622 User's Guide (well back from where the capability is first mentioned on 76):
"If you have another DISH Network satellite receiver installed, you can use this remote in AUX mode to control it along with your new receiver. When you program the remote to control this other receiver, the remote will send out IR and UHF (not UHF Pro) signals to the receiver. You can program the remote to control any other DISH Network satellite receiver, except for following:
o TV2 on DISH 322, ViP222, ViP622 DVR, DISH Player-DVR 522, 625 and 942
o DishPlayer 7100/7200"
Program the AUX button to 3x, where x is the address of the receiver you want to control.
What I take this to mean is that the TV1 Tuner on the 622 is controlled using the same band of UHF frequencies as the older receivers, while a different band of frequencies (called 'UHF Pro') is used to control the TV2 Tuner. So if you set the TV1 address to different channels on your 2 622s, you should be able to do what you want.
Note that this only works with the TV2 remote (the one with the blue key). The green keyed remote is IR only (at least without some modification to the switches that I understand are hidden on the back of the key). You might want a blue keyed remote for each room with a TV.