Prop members are allowed 2 tickets to 2 shows for the entire tour, 4 tickets total. So, if someone wasn't able to use his or her code for the spring leg of the tour, they would have two shows they could attend during the fall leg.
If you were a Propaganda member before becoming a U2.com member and you didn't use your code during the first or second leg ticket sales, then you can get 2 tickets to 2 shows, 4 tickets total. If you weren't a Propaganda member and only joined the fan club starting with U2.com and didn't get any tickets during the sales for the first and second legs, then you can get 2 tickets to 1 show.