Ahh, the arguments of the other thread all over again.
@Quartz just rule it however your table wants to run it.
Apply RT to both rolls (MOST GENEROUS)
Example. Roll 3 and 8, both become 10's. Apply disadvantage results in 10.
Example. Roll 7 and 18, the 7 becomes a 10. Apply disadvantage results in 10.
OR
Apply RT to the lowest of the 2d20s (MIDDLE ROAD)
If you roll both dice, and apply RT to the lower roll, and then have disadvantage result in the lower of the two dice.
Example. Roll 3 and 8. The 3 becomes 10 due to RT. But, because of disadvantage the lower 8 is the result.
OR
You MUST choose one die for RT. (LEAST GENEROUS)
Example. You roll a 6, it becomes a 10 because of RT. You roll
another die due to disadvantage. It is a 2. The result is a 2.
Example. You roll a 15 and RT does not help, so it is a 15. You roll
another die for disadvantage at get a 7. The result is 7.
I found in the course of the other thread that the middle road approach does not alter results
that much compared to the most generous method. So, it is only if you decide to rule the least generous method, that RT doesn't help much once disadvantage is in play.
Just depends on what you want to have more impact? Disadvantage or Reliable Talent.