Hemway-
I have a similar setup to you. I have a 508 and 811. I have a Harmony SST-659 remote and it works great.
First and foremost, you would need to change either the 811 or 501's remote channel that you see in the system info (I use 1 for the 811 and 3 for the 508) The Harmony SST-659 is a very easy remote to setup. it is a web based learning remote through a USB connection. Once you set up you user name with the Harmony website (www.harmonyremote.com), it well then ask you what devices would you like to add to the remote. You would be able to add both devices. Their data bases have tons of devices in it. Including the new 811. Anyway, my harmony remote is setup s0 all I have to do is push 1 of 2 buttons. Watch HDTV (turns on the 811) Watch Dish (turns on the 508)
BTW, those buttons don't just turn the 508 or 811 on. When I select Watch HDTV, one push of a button, Turns the TV on, changes the TV input to the correct input my 811 is plugged into, turns on the 811, turns on the HT receiver and also places my HT receiver on the right input for the 811.
Many ways you can program it. Very cool and affordable remote. Also very easy to use. Most that have it also find their families like it b/c it is easy to use. Hopes this helps,
sorry for the long post, just happy I was finally able to contribute back to a fellow user of this site.