I would hope not. Can you elaborate? I thought U2 prides themselves with their live performance. Now, a backing track during Streets, etc. is acceptable, but not another "live" guitar. Besides the extra guitar sound constitute of arpeggiated notes. I wouldn't think they would get someone to just do singular notes. It doesn't really add to the overall effect of just going live with the four of them.
Coldplay did something similar with Yellow where another guy was playing acoustic guitar in the sidelines. It was to get Chris free to roam around and really interact with the audience. I have mixed feelings about that.