Should you not be exporting them as 8 tracks?
5.1 plus stereo? (or 6 mono and a stereo pair?)
Where are you planning on putting the stereo tracks in relation to the 5.1 mix?
Typically you would have 6 mono tracks L R C LFE Ls Rs and either a stereo pair or (as I prefer) two mono tracks panned for the stereo version.
In Resolve delivery you then set up the individual track outputs under audio, checking that each mono track is listed as mono, and your stereo tracks for 7/8 are listed as you have them set on the timeline, as either stereo or duo mono.
You wouldn’t want to add the stereo mix into your 6 channels of 5.1 mix.
Glenn