migrating from two computers to one

MacOS is inherently a multiple-user system. If you choose to create only one User, that does not change its character.

Mail data for each user is in the ~/Library folder. where tilde [~] is an active abbreviation for "current User". So that is what you want to pick up (along with your keychains file) on First Run.

Migration assistant used later, when applied to User directories, will create a unique user into which to bring over the old data. You will have to manually move the files you want into position.

Anything else can be brought over later with Migration assistant or piecemeal using File sharing. Note that there is no ability to pick and choose individual Applications using Migration Assistant. Migration Assistant wants to bring over all or none. You can always bring over the .app manually using File Sharing, but any libraries might require a manual re-install of that Application.

I expect (but have not tested) that Migration Assistant will not write over a newer-dated Application with the same name, but I expect it will ask if you want to discard older, keep both, or replace newer with older.

You have One opportunity, at fist run, to bring forward your userid, including passwords and emails, into the SAME user account as you had on one of the old machines. you connect the machines and when you get to the point of entering a name for you NEW user account, you will be given the opportunity to import from a previously set up Mac or its backup drive.

Later, you can use Migration assist as many times as you like and bring pieces across from various older Macs, but these will not be placed in your primary account. If the names clash, new account will be intended.

in either case you get the same set of choices as shown in this older version:

Move your content to a new Mac - Apple Support

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