Shimano XT with Campag Centaur crank?
There are a few things to keep in mind. 


Will the current front derailleur work with the new chainring sizes? 

If you have enough adjustment range in the position of the front derailleur,      this can almost always work with a double chainring. However, it may require some grinding of the cage on the front derailleur. The outer edge of the cage needs to
match the curvature of the outer ring and the inner edge needs to clear the smaller chainring when the chain is on the big ring. I think the XT derailluer can work, but Shimano also makes a flat bar component group that might better match
the crank. Shimano Flat Bar Road Group
The new crank will require a different bottom bracket, does the new crank work with the size bottom bracket on your bike? 

Most MTB's have a wider bottom bracket, 73mm vs the 68mm common on road bikes. 
Depending on the exact configuration of the crankset and axle you may be able to solve this with spacers, but the cranks need to clear the chainstays. For a "touring" frame this should not be a problem, my guess is that it would come with the 68mm standard BB width.
Does the rear derailleur have enough capacity to work with the new gears? 

The capacity is (Big - little front)+ ( Big - little back ). 50-34 = 16, 38-26 = 12. That should not be enough of a change to matter.