I have not. Our Armenian parish has had an occasional family or two from other OO traditions in the absence of their own parishes, and I understand this is relatively common.
As for concelebration, that's tricky. It happens, but it's not the most convenient thing for all involved. Remember, each OO Church has its own rite, differing language traditions, etc. etc. For an Eastern Orthodox clergyman, say a Russian priest, going and serving in a Romanian or an Albanian parish, or a Greek priest in an Antiochian or Russian parish, there's not much difference there. Linguistically, there may be some problems, but at the end of the day, it's all extremely similar.
It's not like that for us in the OO world. And our relations have suffered as a result. At the same time, I think it's extremely valuable that all of our respective churches have retained our own traditions and differences, yet have also retained our unity.