Merging two Multisites

I'm asked to merge two multisite installations. The goal is, to get all the blogs hosted on *.server1.com over on server2.com Each server has 40 blogs to start out with, so server2 will end up with 80 blogs.

I'm interested in hearing from anyone who may have tried something like this, and to get their general ideas on how to best approach it, or to hear about any experiences others may have.