When someone sends you a text message, changing the subject, before you have the chance to reply to the previous one. By the time you reply to both, your conversation is now more than one topic. It's like talking to 3 or more people, but it's really only one.
"I'm getting a headache from double texting subjects. I can't text to my daughter about one thing at a time. Before I know it, she's texting about 5 different things and it's difficult to keep up with what we're talking about".