According to your descriptions both clients are connected locally to the same storage, one using the internal harddisk and the other an attaches USB drive. Only one client is active at a time (never both at the same time), so simultaneous write-access should not happen.

I cannot give any guarantees for the following, since I've never tried (and also will not try, at least in the nearer future), but using symbolic links, the following should be possible -- provided the userIDs (of the user on both machines) match.

Assumptions:

Let's call the machine with the "internal disk" local, and the one primarily using the USB disk external. On both machines, your user is willie. On local, your home directory such should be /home/willie, and let's assume the USB drive is mounted at /mnt/usbdrive with willie's external account such being at /mnt/usbdrive/home/willie when looking from local.

Setting it up:

1. make sure userIDs match: when logged in as "willie", on each machine at a shell prompt enter the command id. Output should start with uid=1000(willie) gid=1000(willie). Make sure uid on local matches uid on external, same for the gid. Otherwise you will certainly run into a permissions problem.


2. just in case: make a backup on local (on external, it cannot hurt either). You might want to run an export from Evolution, but at least do a cp -R ~/.local/share/evolution/mail/local ~/backup/mail_local while Evolution is shut down (of course, you might want to use a different target).


3. now remove the local mail on local: rm -rf ~/.local/share/evolution/mail/local


4. and create a symlink from the USB disk: ln -s /mnt/usbdrive/home/willie/.local/share/evolution/mail/local ~/.local/share/evolution/mail/local

Conclusion:

That should be it. For willie@external, nothing has changed -- so everything should still work as before. For willie@local things might be a little different: local mail now comes from the USB disk (if Evolution accepted the symlink). Provided local mail doesn't use any "external configuration" in ~/.config/evolution/mail, it should work. Hopefully, it uses only the symlinked directory (it certainly does for folder organization, as it saves its folder.db here).

Good luck, and please report back! Though I do not intend to have use for this kind of solution in the near future, others might :)