If you add both a GTN750 and an RXP GNS530..

.. beware, because the GNS will likely be selected as the "Active navigation device" when loading up the the airplane.

You can mouse over the memory card slot and Tooltips will tell you, or, when powering up the GTN, you will

notice that the HSI does not deflect properly when on the test page.

In that case, pop up the GNS and right click on the right hand memory card slot to toggle the device setting.