It depends. I have a pair of proven male bristlenose (also only about 4-5” long). The elder was from a packed tank so he was used to sulking away while the younger was a good 2” shorter and was used to being alone. Got a little violent the first time they ended up in the same cave during a water change but I’ve seen them a couple times pretty close to each other. Close enough they know the other is there anyways. So far no scrapping and they have half a dozen caves between the two of them. It’s also a 65g tank and they have other loaches to scrap with.
That being said, for a 20g, I absolutely would not have 2, becuase even a 20g is pushing it for the smaller plecos. Do you have a picture of yours?