I still haven't figured out if JP is actually doing backup vocals or not. Looks like he mimes.
A couple of things on that:
1. JP denies lip synching and says he sings every backup vocal, but intentionally keeps his backups low in the mix because he doesn't really like the way he sounds, and does the backups both for the visual and for just adding a bit of different texture. At least, that's what he said back on the DT12 tour. I haven't spoken with him about it or heard him comment since then.
2. Whether or not #1 is 100% true 100% of the time, from my experiences with the band, I can vouch for it to an extent. On the DT12 tour, I was close enough to JP to hear him singing (probably about 5 feet from him). I have also heard them sound check a few times where they run through songs without James before he comes out, and I will tell you that, as others have noted, they have James' voice piped in doing harmonies on the backing vocals for parts. Those backup harmonies are higher (louder) than JP's mix. So what you get in a live setting, at least in some parts, is James singing lead, piped in backing vocals, and JP singing, but almost buried beneath James, the backing track, and the instruments. That is what I witnessed firsthand on the DT12 tour (and I think I remember being a able to hear JP at the second TA show in Reno where they had a standing area/pit right in front of the stage, but I don't remember for sure anymore; Geeemo may remember).
That info is a bit dated, but I think it is probably likely that that is what they are still doing.