Teens on screens: Life online for children and young adults revealed – Ofcom:

This year also saw the rise of ‘split-screening’. Split-screen social media posts allow children to watch more than one short-form video simultaneously, on a single-screen, side-by-side or stacked on top of one another. This appears to be a progression of the ‘multi-screening’ behaviours seen in previous research waves, where children reported difficulties focusing on one screen-based activity at a time. 

Distinctions needed here: There is a difference between genuinely watching “more than one short-form video simultaneously” and merely having more than one short-form video on one’s screen at a given time. I seriously doubt that it is possible for any human being to watch two videos at the same time; the best we can do, I suspect, is to switch rapidly between two videos, and a good deal of research indicates that we’re not good at doing even that. Every time our attention switches to one information source we cease to attend to the other.