A merge member add-on would be nice
I would love to test this!!I made a undelete script that lets you change the threads and posts from 1 user to another.
It's mainly meant to enter the id of a new username, and then the name of the guest posts ..
It's not properly tested, but in effect you're merging users. Moving threads posts from an id to another id, and then after cache rebuilds etc one of them should have 0 left, and the other should have a lot more. You could then just delete the one with 0 left.
Let me know if you want to test this on a test install of your site to see if this helps you merge users.
It worked on 2x 1.1.2 sites - but they were quite empty and were a fresh install.