Unusual Drain Problem. What to do???

I have a 1964 duplex. For the last few weeks water wouldn't go down in the bathroom sink, then it got worse, gunk started filling up the sink when I wasn't even home. The bathroom in both units are back to back and obviously share some plumbing since their water won't go down the BR sink either. I tried the salt, baking soda and vinager flush 3 times. It didn't help at all. I had a plumber out finally, and he said there was no visible way to remove the debris, and that a snake would just push up into the other unit sink, not down the pipe and in summary my Home Insurance Policy won't touch the problem. He said I might have to go through the roof to fix this. Help!

It sounds as though the two bathroom sink basins drain pipes are attached using a cross. Oftentimes, a drain snake will jump straight across this fitting, and come up in the other drain.

Chances are, the vent pipe for these sink basins carries out through the roof, and the drain line that is clogged can be snaked out from there.

It sounds like a job for a proffesional, as climbing up on roofs or around in your attic isn't something I would reccomend. And make sure the plumbing company has valid insurance.