There is a sadako vs kayako movie, basically The Ring vs The Grudge, and from what I gather the curses can combine into an even bigger curse. On the other hand in sadako vs bunshinsaba (the Red Lady) they basically just brawl until they're both trapped forever.

I get the feeling that the It Follows creature would suck in a fight, but it would make for an excellent post credit scene where it walks into a well.