One has a forward loop with a thunderbolt in the middle, and the other has pages stacked against each other.
Sometimes one or the other flash, sometimes they both flash, and sometimes they flash rapidly or just don’t flash very often.
They always appear in the upper left of the hero select screen.
What are these symbols and what do they do?
According to What the connection symbols mean,
The lightning bolt means that the server has not heard from your client for some period of time and is now extrapolating.
The squares mean that you’re experiencing packet loss.
Source (Jeff Kaplan): http://us.battle.net/forums/en/overwatch/topic/20742919566#3