I know the answer to this question is probably no, given the respective differences in audience size between late night on cable and late night on one of the big three networks.
However,
I think of a good percentage of Colbert's audience as moving on to Conan's late night audience in the past. I think of the two as appealing to a similar audience, and I imagine there being some overlap. This week, Colbert is doing his USO thing which is kind of a big deal for his show. Could that be taking some of the Tonight Show's audience?
Personally, I've found Colbert to be lacking in humor lately. But his USO gig has been fantastic this week. I've been thinking of switching to Conan over Colbert for the 11:30 timeslot, but it's going to have to wait until next week.