(Looks like Girk and I were typing at the same time. Hopefully this will clarify a bit more).
The IR signals are NOT all the same. Your question was can you use D* and Harmony remotes at the same time. Yes, but code 1 controls a receiver set to code 1 and no others, code 2 controls code 2 and no others, etc., no matter what remote the signals come from. If you have remotes A and B set to code 8, remote C set to code 1, and one receiver set to code 8, then that receiver, and only that receiver will respond to remotes A and B (at the same time). It will never respond to remote C.
There seems to be a lot of confusion about this. Maybe you should think of the codes as different channels. Imagine you have a pair of walkie-talkies, one representing an HR20 and the other representing a remote. Set both to channel 1 and they can talk to each other. ANY other radio you might buy and set to channel 1 can talk to your other radios on channel 1. Now set one of the radios to channel 2. It gets nothing from your channel 1 radios. If you change your other radios to channel 2, they can all talk again. Does that make any more sense?
Codes 1 and 3 are the only profiles in the Logitech database. You have to teach it the others.
If you are interested in the nitty gritty details, I have analyzed the signals for all 8 code sets. There are a few bits near the beginning of each signal that identify which code set it is. The rest of the signal is identical to all the other code sets. In other words the signal for, say, the Menu command is the same for all code sets except for a few bits at the beginning. When a receiver gets a command and those first few bits don't match what it's looking for, it ignores the signal. If there is a match, it executes the command, like a lock and a key. Only matching sets fit.
Edited by mdavej, 29 July 2009 - 08:05 AM.