Somebody mentioned Dallas could play Bono's parts. I don't think Bono's parts were important enough for anyone to need to stand in and play. Plus, as somebody else said, Dallas really does have his hands full with Edge's gear. He works Edge's pedals if Edge is away from his pedals (he has a set of pedals in his area under the stage), gets the guitar ready for the next song, has to be ready at a second's notice to swap out Edge's guitar for a replacement if he breaks a string or the guitar just doesn't work for some reason. He's a busy guy!
Fun fact about Dallas. At the Sydney 1993 gig that Adam missed, one scenario they considered was Dallas playing guitar, and Edge playing bass for the entire show!! They weren't sure if Stuart Morgan could do it. But they realized that 1: that would confuse the camera crew, who were using that gig as a rehearsal for the 2nd show's broadcast and eventual live release, and 2: a crowd at a U2 show expects to hear Edge on guitar! So that must mean that Dallas knows how to play all the songs, in addition to known the effects they require.
My source is Flanagan's Bible "U2 at the end of the world". Read it immediately if you haven't!