I use the back to back uni like jig n pig. I tie/have tied 10lb powerpro to 10lb mono, 20lb powerpro to 17/20lb mono and 30lb powerpro to 12-15lb flouro with no problems.

I thought that the different diameters of the 20lb pp and 20lb mono might be an issue but when a 20lb salmon put it to the test, I put him in the boat :D