If the mx-900 has the tivos set to alternate addresses, but the TiVo's are both set to address zero. You need to cover one, has you make you way through the TiVo menu to INFO screen, pressing a key on the remote set to an alternate address will switch that TiVo to same alternate address. Get out of the info screen, now cover up that TiVo, and repeat using a different device on the mx-900 set to a different address.

If its the opposite, and the tivos are correctly set with the alternate addresses, and its the mx-900 that has to be programmed. In ccp software, under database, go to DVR, then Tivo, you'll find addresses in 22? code range.