Haste doesn't do that. It lets you take an action that only lets you Attack(one weapon attack only), Dash, Disengage, Hide, or Use an Object. Nothing in there let's you cast a second cantrip.
How are you maxing them all? I assume you are using Overchannel. It's...technically legal to use it on cantrips. Though Mike Mearls said that it was legal "as written", which implies that he doesn't want to issue errata without discussing it with the rest of the team. He then proceeds to say that even he would house rule it to increase the damage by 1d12.
I would just not allow Overchannel on cantrips.
Well, it is legal. You would be house ruling. Yes, Haste they nerfed from Alpha, I missed the change where they moved cantrips. Never the less, they can cast a channeling damage spell, and spam cantrips very easily for high levels of sustain.