If the game against RSL was a regular season game and then the game at Atlanta was a playoff game, then I get sitting the starters. And I do agree that the game at Atlanta is more important for us as they will be vying not only for a top spot in the East, but also for the CCL spot.
However, I don't think I would sit our starters against RSL (that said, I would be cautious with those returning from injury). If it was as simple as a math equation that if we play our A team at Atlanta that we will get a win, then do it. But its not, Atlanta is tough and we may not win there with our A team. If we go B against RSL and then A at Atlanta then we are risking dropping a decent amount of points.
Start the A guys against RSL and then assess fitness for the game at Atlanta. As
Gotham Gator points out, this is 2 games in 5 days with a week or more on each end. This is very different to the scenario we encountered last year with 3 road games in 10 days.