The situation where you begin having a texting/imessage conversation about 2 separate topics and your messages go back and forth like a swirling DNA helix - i.e. discussing your specific weekend plans and also an depth conversation about The Avengers movie at the same time
"My mom brought up weekend plans, which caused us to be multiplexting for a while."
Portmanteau of multiplex and texting.
My phone lost connection for a couple of minutes, so when my text was finally send, she had already switched the topic and we started crossconvoing. So annoying!