Here is what I do. I hookup the 522 through the 721's front ports and turn it to channel 0 to tune in the 522. I then press the red "i" button to have two tuners running at once. All I have to do is press swap to go back and forth between the 721 and 522 ports while still being able to use my S-Video/A-V cables to get the best picture and audio quality. You can then press DVR on the 721 remote or 522 remote to access whichever DVR list you want. This gives you a total of 190 hours of shows that you can watch.
I like the fact that the 522 allows you to choose a channel for UHF use which brings a better picture quality, has extended UHF remote functionality since it uses UHF Pro, and has two tv outputs. It also allows DVR functionality on two seperate televisions, saves $5 in DVR fees and $5 in additional outlet fees.