Also, if the characters have a DNI link to their commlink (datajack, trodes or technomancer), then they can control it with their mind. Sending text messages and updating tactical information would be as fast as thought.
There is a piece of fiction, I think in the first SR4 core book, where a bunch of runners in a meeting with a Johnson go quiet for a bit (from the Johnson's perspective) to mutter to each other over their subvocal microphones about whether or not to take the job.
The main issue, I think, with trying to AR text chat with someone and talk to someone else would probably be distraction from multitasking. There might be some pauses. Then again, having stuff going on in AR 24/7 is pretty much the norm in the 2070s, from what I understand.