I have no experience with tandems, but I have a lot of experience with X517 rims. They're a lightweight mtb rim, considered pretty much a racing rim because of their weight. The X517's have held up great for me, and in fact I built one of the wheelsets that I have these rims on and have taken a lot of pride in the fact that the wheels have held up so well despite thousands of off road miles on them. BUT, and again I have no experience with tandems, they'd be one of my last choices in a 26" rim for a bike that had to carry two riders, they're just too light IMO. I think Sun Rhyno Lites would be a much better choice, or one of the newer Mavic mtb models that are designed for "all mountain" or even "freeride" duty. Of course, the number of spokes is a big factor, too. How many spokes does the wheel you're going to build have? My X517's are 32's, I would think at least 36 would be a good idea for a tandem, if not more-