There are a few thousand differences between Vaticanus and Sinaiticus. This is a point that MT advocates think helps their cause. Actually, it hurts them. Here's why: (a) Westcott and Hort noticed those differences, too, and argued that precisely because of so many differences the common ancestor between B and Aleph must be at least ten generations back. They felt, with good reason, that the common ancestor came from deep within the second century. Consequently, when the two MSS agree, their combined testimony should normally be regarded as quite ancient.
Majority Text advocates like to tout how much Byzantine MSS agree with each other. Yet, they also want to claim that each Byzantine MS is an independent witness to the text. They can't have it both ways. The high level of agreement shows that there has been extensive editing of the Byzantine MSS. Indeed, we have fairly firm evidence of such activity in the 9th and 11th century, for after both eras the Byzantine MSS grew in their conformity to one another. This is unheard of except when conscious editing takes place. Further, some MT advocates want to claim that Aleph and B were copied in the same scriptorium and that they have a common ancestor that is not much earlier than either one of them. How can they claim this while simultaneously noting the many disagreements between these two MSS?
We do have an example of two MSS that copied either from the same archetype or were at most one generation removed: Codices F and G in the Pauline corpus. I have examined both MSS in the flesh, and collated large portions of both of them. They are ninth century documents, whose ancestor can hardly be any earlier than 8th century. The patterns of agreement and disagreement are completely unlike what you have with Aleph and B. I would concur with Westcott and Hort that the common ancestor between these two MSS must be at least ten generations back. I hope this point is clear.