Televue make an 8-24mm zoom eyepiece with click-stop positions. As I understand it, the click-stop at set focal lengths is precisely to make them easy to use in binoviewers. Many make 8-24mm zooms. I have a Seben (public viewing) and a Williams Optics. Nether is click-stop, but I believe some others are, apart from the TV. Note Field of View is small - some are wider than others - some are more parfocal than others across the full zoom.
http://www.televue.com/engine/page.asp?ID=274 (Seems to be $299 on Bintel's website - not including Dioptrix correction lens, if interested in that)
I tried a binoviewer once on my previous 8" newt. Bit hard to pull focus with some eyepieces - I think I had to add a 2x 2" barlow into the train. Clearly less light through each eyepiece. I handed it back pretty quickly. When I go two eyes, it will be a big binonewt!