Although it's suggested to use two different kinds of locks, I doubt if there were many cases when one used two same kind of good locks (e.g. two U-locks) and the thief actually cut/defeated both of them. Anyone heard such cases? or perhaps practically it's still as good as two different locks (a U lock and a chain lock)?

Yes, in my city there is an online registry of stolen bikes. People put down how it was locked. Plenty of them say the bike was locked "to itself" . they don't need a car just two legs to carry it away.

Has anyone used Pitlock? I have OnGuard skewers for my tires, but I was thinking of getting a nice bike for next summer and keeping my current mtb for winter. Pitlock has locking bolts for just about everything and was thinking if i got a nice bike i might use those. of course it's no guarantee.

I read a sad post once (I think alt.bike* when everyone was on usenet) by a guy who came out after lunch to see someone had cut all his spokes and stolen just his hub!