I cannot vouch for every writer or artist, but I have seen usually only one pen or fingers on keyboard, working on one book, one page, one line at a time. I have only seen an artist use one brush, one stroke at a time. This is not to say that others have not been able to do some incredible works with the Olympic medal of multitasking brilliance behind them. In saying that for people like me, it may work well to just use one brush.
That also requires a sort of slowing down. And even though it may seem a bit challenging when the phone is ringing and everyone has a request for you, let them leave a message and finish what you've started, one moment at a time.