U2 have only one prime guitarist. They usually use Bono for rhythm guitar in the studio and he is very talented. But when you are as enegertic as Bono then you don't always have time to play the guitar at live shows. So this kinda explains that. However, in saying this, Edge is a great guitarist and older U2 songs i.e Streets, With or Without You and I will follow are all played on the one guitar by Edge. But the bands new stuff, really does require more than one guitar and its not so much as a rhythm its more like two lead guitars.
BTW, if you liked Boston, check out Zoo TV, the ultimate rock show.