Ethernet or telephone?
Phone, you just need a splitter.
Ethernet - something like this at each end of a single cable. (I'm sure cheaper versions are available..)
Edit: Duh. I should read the post title. Unplug the RJ11 from the Tivo. Plug it into an RJ11->BT socket adapter. You can use a spare ADSL filter, using the modem & phone connectors and leaving the plug dangling. Plug a standard phone double adapter into the phone socket. Plug second tivo into one side and new modem cable connected to first tivo into other.