Assuming your rims check okay and spec to size, Kenda Tandem tires are 26 x 1.5 rated for 100 psi. I have run a set. They're heavy but great for gravel trails, and they're okay for the road if you're not a speed demon. I bought mine from Precision Tandems.
I'm doing an Adventure Cycling trip on the GAP/C&O Canal in September and I'm considering using them on my hardtail MTB for the tour.
http://www.precisiontandems.com/catframepart.htm
http://www.precisiontandems.com/cat_pics/trkenda26.jpg