Merging accounts only happens when someone has two (or more) accounts for any reason. Usually because they didn't know that two accounts isn't allowed on iwaku. I'm not sure how the process works, but I'm guessing Diana lets the one needing their accounts merged choose which username they should keep. (Otherwise it's just to change it in the settings after the merging is done). As for the rest, I'm guessing things like the rp resume or account details can't be merged into one another since there's only one per account, thus you should logically keep the one of the account which name you've kept. For things you can have multiple of, like followers/following, blog posts, forum posts, groups, they will all most likely be moved to the account the person has chosen to keep and then everything will go beneath that username instead of the other one.
I'm no expert on the subject since I've never had accounts merged nor been a staff member that has done merging, but I can imagine it would be something like that. If someone needs to get their account merged I'm sure Diana tells them what will happen and if the person needs to save something if it can't be merged into the other account.
