You can do it manually. Close eM Client and copy the Archive folder from one to the other.
This is going to replace what is in the target machine archive, so your best option is to only archive on one computer, then copy it across.
Another option is to not use eM Client’s Automatic Archiving, but instead manually archive to an IMAP or Exchange folder that both computers are syncing to. Kinda defeats eM Client’s definition of archiving though.