i got a 03 ultra classic bagger with duals and the crossover pipe
Q..can i block off the left pipe and use the crossover as the other header for a 2 into 1 exhaust.

 

I am pretty sure your crossover is so each cylinder uses both mufflers. Harley is a four stroke that fires ever two strokes. Crossover is just a buzz word along with 2 into 1.
You actually have 2 into 2. (which is better then 2 into 1) To answer your question, yes but you would be double back pressure. Would not notice it on bottom end but would cut power at higher rpm. However are you talking about adding one larger 2 into 1 muffler?

 

i'm runnin straight pipes now but was thinkin of addin the larger muffler if that is correct