A couple of years ago Gary & I did the 2 Barlow thing. If I recall correctly we coupled up a 2x Barlow with a 2.6x Barlow, giving us essentially 5.2x, but it was actually more due to the positioning of the Barlows. Worked ok from memory, but we only ever did it once, so it can't have been that good. I think it was on Mars. Since then both Gary & I have aquired 2.5x Powermates. It would be an interesting experiment to couple them up one day!
Cheers