You could. I don't advise it. The older bar will have a different rate than a "new" one. Just like putting on only one shock or one spring. The spring,
say a leaf style would sit different.
If you are saying. Using a new used one. Make sure it's from the drivers side.
I have seen guy's put one in from the other side and then snap that one.
Just because in was use to bending the other way.
Some will say different on that one.