Hard to say with out seeing you on the bike, but it sounds like you are "rolling" too far forward on the saddle. Check your position on the bike and make sure that you are able to truely "perch" on those sit-bones.

It could also be that your saddle is too wide in the front.

Don't be surprised if you find you need a different saddle on the tandem than on your single bike. Try searching the saddle forum for other saddles to try.

I can ride the same saddle, single or tandem, but find that shorts that are comfy on the single irritate me on the "big bike".

Weird, huh?