tuning the obs on the rmi

on those aircraft equipped with the RMI, how is the obs or course selector
moved to use the vor indicator as nav2

Think of a RMI more like an ADF than a VOR. The heading dial is slaved to the compass and the 2 needles just point to the stations selected. For the number 2 nav, it will point to the VOR station. It works well in a situation such as identifing an intersection. For example, you are on the ILS inbound on the number 1 nav and the FAF (final approach fix) is the intersection of the ILS course and the 247 deg radial of the ABC VOR. Set the number 2 nav to the ABC VOR frequency and when to pointer points to 247 degrees, you are there. One trick I like is when there is a VOR on the field. I set the number 2 NAV to that VOR and I know when the needle comes within 5-10 degrees of the desired runway heading, I can turn final and should roll out aligned with the runway. As for the ADF portion of the RMI, I set up the ADF frequency to the LOM (localizer outer marker) on the ILS (when available) and the needle should flip around as you cross it (while the blue marker light flashes and beeps). Often the FAF is the LOM and it is also a good way to identify it. Give it a try with your GPS off and see just how well you can find your way around.