Here is the other side, so you can see it is healthy-looking.

Do willow trees split? I just realized that it looks like there are two trees right next to each other at the base of the trunk. Could it be two? Or do they look like that?

It's a 'corkscrew' or contorted willow.
That's what they do...rapid growth, then unsightly die-back.
I have a couple here that look not unlike that.
IMO, they're best as a coppiced specimen - whacked back to ground level every 2-3 years, fostering regrowth of multiple vigorous contorted shoots.
Left to grow to 'tree' status, you end up with an ugly thing with extensive areas of dead limbs.

Ok, thank you! I love it too much to chop it down, but I'm definitely going to remove the dead parts. Good to know it's fairly normal and that it's not just dying.