I currently have a H890 and the display is starting to give me problems. (grey display, garbled text, lines, etc.). I use this remote to control 2 receivers. They are both in a cabinet downstairs. One controls the home theater and the other controls audio to the living room and my office, so I need the RF function. I would like to use the 1100 downstairs and take the 890 upstairs to control the audio. My question is can I use both remotes and use just one RF wireless extender, or do I have to get another RF wireless extender? Has anyone tried this?

No, you can only pair one remote to the RF Extender at a time

Not the answer I was hoping for. Can you explain why? If both remotes are programed with the same codes does the extender just pass the signal on or is there some logic involved in the remote?

There is only one Harmony remote that is able to have up to 6 remotes paired to the same extender and that is the 890 PRO.  I don't have an answere as to why only that that is the way Logitech has designed it to be.