That's confusing. 1 database, 2 instances?
Anyhow, just create a linked server entry. There really is no need for an ODBC setup for that task
And you specify the instance by name. Either if it is the default name or a named instance. Meaning when the DB Server was setup you either left it the name of the actual server or changed it to something specific.
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The worst mistake you'll ever make is to do something simply the way you know how while ignoring the way it should be done