We have limited capacities for attending to what happens around us. Two or more objects are not being held at the same time in memory. We switch between them. Once we have copied them from long term or permanent memory into working memory then short term, we can find those copies pretty quickly again. The more complicated the behavior and the more dissimilar the two or more tasks, the worse we multitask. By worse, I mean we are more prone to error and take longer time.
Given all the research and media coverage on how bad we are at multitasking and risks associated of texting and driving, I see people doing it daily. A law here allows police to write tickets to people who do it for the past few years. Texting is also pretty bad even on hands-free devices. Like other risky behaviors, these approaches are unlikely to stop humans from putting themselves in dangerous situations.