I created a nether1 world and linked portals from another world to there, then the portals in nether1 back. It worked at first for me as op but then when someone else tried they got a "This portal goes nowhere" message trying to get back from nether1. Debug mode showed that it was attempting to send them to nether1_nether even though they were in a nther world and the portals were set to link to another world all together. after a couple tests it even stopped working for ops. Sometimes creating a second portal in the nether map would link back to the first portal in the other world, sometimes it wouldnt, error was always that it was attempting to send to nether1_nether though.

Probably fixed: http://multiverse.github.com/news/better-end-support.html