Merging on multisite setup only displays user for current domain (UFMatch)

When two contacts are de-duped in a multisite setup, if you choose to 'take across' the other contact's user account, it removes any other user accounts on the main contact.

eg:

Mr Main Contact
UFMatch: Domain 1: User ID 1

Mr Other Contact
UFMatch: Domain 2: User ID 1

Merge, choosing to 'take across' Mr Other's user account.

Expected result:
UFMatch: Domain 1: User ID 1
UFMatch: Domain 2: User ID 1

Actual result:
UFMatch: Domain 2: User ID 1

Proposed resolution:

The merge screen should separate out UFMatches by domain, eg:

Domain 1 user account: username 1 =[ ]=>
Domain 2 user account: username 2 =[ ]=> username x

This should not be too obtrusive for the majority of sites which do not use the multisite setup. It would also make the merge screen more descriptive about what the 'move related... username' actually means.

Currently the user tables are just replaced with those being kept. So, in this example:

Contact A (other - to be deleted)
UFMatch Domain 1: UID 1
UFMatch Domain 2: UID 2

Contact B (main)
UFMatch Domain 1: UID 4

Merge, taking A over to B

Expected and actual:
UFMatch Domain 1: UID 1
UFMatch Domain 2: UID 2

And the UID 4 match has been removed from the UFMatch table.