I have always wondered if it couldn't be something as simple as two scribes
copying text from a draft version and inserting some null characters on the fly
as they copy. Differences in their preferences in the choice of nulls and the
frequency of inserting them could account for the difference of the hands. The
intermixing of hands by page, but never (I believe) within a page could be the
result of each scribe grabbing a page or two of the draft, copying them, and
coming back for more when he is done.
One way to test this hypothesis, it seems to me, would be to look for a small set of characters (the nulls) which, when they are deleted, cause text of both hands to have similar characteristics (e.g. word length, letter frequencies, word frequencies, etc.)
I tried some testing along these lines a few years ago, but didn't find anything conclusive. With today's fast PCs, though, it should be possible to automate such a search, I think - maybe I'll take another look at it.