The theoretical probability having been correctly stated already, then the pragmatic probability can only be determined through empirical observation. And, it will constantly change with the environment. Every two computers will be different.

If you simply reduce your problem to “two adjacent words in a data-stream,” you are now talking about the same sort of problem as describing the frequency-of-occurrence of “digraphs” (two-letter groups) in a classical cryptography-type scenario. Once again, it depends upon the particular data-stream (e.g. the native language of Alice and Bob).