I was looking for the same sort of thing, and came to the conclusion that nothing exists to do it. I'll be curious to see if someone knows of something.

Alternately, perhaps you could get two clamp-on cable housing stops and put them on the frame on either side of where you want to split the cable, allowing you to just have the bare cable there. Like this (** is the housing, | is the clamp-on stop, ---- is the bare cable, and X is the splitter).

**********|-----X-----|************

For a while I had a cable splitter on the front v-brake of my tandem. The only bare cable to attach it to was in the space between the noodle and where the cable attached to the anchor bolt. It fit, but barely. I finally did away with it because I figured it really wasn't that hard to detach the front brake cable and get it quickly dialed back in when reassembling.