Join two pipes with independent rotation?
When you say joining two pipes, do you mean at right angles, or axially?

You could use a ball bearing, with the outer race of the bearing press-fit inside one pipe, and the second pipe press fit inside the inner race.   A large linear bearing would work too (but cost a lot).

Do you have access to a lathe?   A slip fit between two pieces of delrin or metal and delrin might work---all the gearing and linear bearings in some modern printers and scanners work much like this.