You can set up 5.1.2 for monitoring within the Production Suite in the Room Setup page. Enable Ltm/Rtm in the Speaker Setup tab and then create a 5.1.2 monitoring layout in the Monitoring tab with L,R,C,LFE,Ls,Rs,Ltm,Rtm selected. It will then be available as a choice in the Monitoring drop down menu from the main Renderer window.
There is not currently a dedicated 5.1.2 re-render but this is planned for the next release.
If 5.1.2 is feasible in your space than I think it would be of benefit to install Ltm/Rtm speakers. Having two overhead speakers would certainly be more representative of what your Atmos mix will sound like when transferred to a 7.1.4 stage than not having them.
Adam @ Dolby
Thanks Adam. That info helps quite a bit!