You can have any # of remotes to control your receiver, something's wrong.

It'll be able to control it in single OR dual mode. Does the "chip" in the bottom front of the 21.0 say "1"? If so, get the manual out and read/follow the instructions for setting the remote address again. If not, remove the battery cover 1st, then turn the "chip" over from 2 to 1 and reinsert it. Turn on the TV in the living room that's by your receiver. Turn on the receiver as well.
You need to set the address with the new remote to your receiver by:
1.Press the INFO button located behind the door on front of the receiver.
2.Press the RECORD button on the new remote (RED).
This should syncronize the receiver with the new remote.
Yer welcome!
Ed
That should automatically set up the remote with current TV1 address and change TV1 from IR only to IR/UHF so that both the new 21.0 and your oiginal 20.0 remote will control TV1The 20.0 IR remote will just work in the living room though--as it has.