Make a second table occurrence of Item Master and use it for the second relationship:
In Manage | Database | relationships, make a new table occurrence of Item Master by clicking it and then clicking the duplicate button (2 green plus signs). You can double click the new occurrence box to get a dialog to appear where you can rename the new occurrence box.
We have not duplicated a table. Instead, this is a new reference to the same table already present in your database.
You may also want to using a different approach than two separate replacement fields. You could define a 3rd table to link to both x-Ref Master and Item Master with one replacement item listed in each separate reocrd. This results in a simpler relationship structure and also allow you to list any number of replacement items. You are no longer limited to just two.