I cannot confirm that Airbus plan a double slide for the upper deck. The 747-400 slides are. I would be surprised if they were not planned to be so because of torsional twisting problems- it would be harder to keep a long slide like that untwisted in wind. The nightmare video of the Pan Am SFO take-off incident where extensive hydraulics failure caused a tip back after landing and the slides flailing in the downdraft of the helicopters buzzing around filming with people falling off them must never be repeated, not from that height!
I actually think the upper deck will be a slightly safer place to fly. You have more 'padding' below you. I'm familiar with the operation of the upper deck slides and have no doubt they are virtually as good as the lower deck ones. I would assume the same manufacturers will produce them and they have the benefit of 20 years progress in slide technology on the -400.