I agree - that with RAW this argument doesn't stand, but RAW also says nothing about characters having the ability to breathe. Interpreting the rules then --- characters can't breathe. And RAW states follow suffocation rules - and then we won't be having this conversation. My dislike for RAW is another topic. I'm sure there are plenty of headscratching involved when running the game as RAW.
But I still maintain that the game is intended to be run with characters being able to breathe (even without the express RAW that they are able to), and that sometimes we just have to RELY on our DM's to make judgment calls on fuzzy logic, and hopefully still have a blast doing it.
This is a fundamental misunderstanding of the role of the game's mechanics (rules) in running the game.
In D&D (and any other RPG I've ever seen) there are some things the rules
do cover, and some things the rules
don't cover.
For the things the rules don't cover (like....breathing without being suffocated, eating, shaving, excreting....), that's what the DM is for. The expectation is that these things work just in the game world just like they do in our world, and the DM is expected to rule as such if it ever comes up in play. These things are also not covered by rules because they are not interesting in the fantasy genre.
But for the things the rules
do cover (moving in combat, attacking, etc.), then the rules are permissive; meaning they do what they say they do, and you can't claim that your 1st level fighter can cast
wish at will on the grounds that there are no rules that say I can't!
As a rules question, yes, you can Dash twice per round if you can Dash as a bonus action.
So SUMMARY - RAW, double dash looks to be allowed. RAI - fuzzy for me.
RAI has also been solved. JC has answered this specific question, and yes they can Dash twice per round and yes this is intended.