Merging two existing multisites

I need to merge two existing multistes. Each has some 40 blogs, and the end result should be that all 80 blogs reside on one WP installation.

Any advice or experience on how to best apporach this task, other man doing it manually, by creating each new blog and user on the end server, and then use export/import for each individual new blog?

I’d like it to be an efficient method, that could also be applied on much larger multisites. Direct DB manipulation and Script automatization..

I’m interested in paying for assistance, if anyone has proven experience, or can suggest a good solution.