When on the system info screen (Press Menu twice) use the 21.0 remote to set the remote address. Note on the screen if it's UHF A or UHF B. Then on the 6.3 remote open the battery cover and make sure the slider switch is set to A or B to correspond with what the 21.0 is. Program the 6.3 to the same address and try, they should both be working now.