2 characters at once would be risky. They tried that in HotS and it didn't go so hot.
This is pretty cool, whatever they are doing.
Zarya, McCree, & Mei were simultaniously revealed/released during the beta, and it really threw everyone in a loop. While I wouldn't mind having more than one hero shown, I'm quite fine with the hero releases been spaced apart.
If anything, I want more maps, along with switching the day/night cycle.