1. as mentioned, easiest solution, although not multiplexing, is to replace the return video fiber with a set of one-strand gigabit media converters to carry your PTZ data.
2. if you actually need the return video, then next easiest solution is to upgrade your fiber cable to 4 or 6 strand cable, using the extra strands for ethernet MCs.
3. If you are hell bent on using return video and sticking with 2 strand cable, then you will have to "roll your own" solution with optical splitters/combiners and devices with other frequencies of laser to share the glass with what you are already using. there are a few pre-built solutions that don't include the talkback function of the BMD converters, but will get you video both ways + ethernet.
the second solution is what we live by, all of our cable is either 6 or 12 core fiber. we just bundle gear that we need in a location into a rack case so there is only one fiber cable to run between ends. VERY time efficient when it comes to setup/teardown.
as for what all do you want signal wise in fiber? we have created many systems that transport many 3G-SDI video in both directions, ethernet via managed switches (multiple networks), and audio/comms channels for mics, ifbs and intercom.
pictured here, 6 strands of fiber gets you
3 in and 3 out 3G-SDI video
6 isolated ethernet networks, with 2GB of bandwidth between ends
32in and 32 out audio, 24x24 for general audio needs, and 8x8 for 8 discrete channels of intercom belt packs
extension of one matrix intercom panel (RTS ADAM-ZEUS)
the top box goes near your control room, the bottom box goes in the field/annouce booth/wherever you need signals. the 6 little red caps on each box is where the fiber goes between ends.
we've been using this system for years with no issues, on some pretty high profile network broadcasts
it's pretty cool what you can do with fiber and a little imagination.