The whole morning I tried to find the part where Henkka took over, but I couldn´t find it either in the youtube vids or the stream Steve recorded, sorry. There are a few parts where Alexi´s voice is missing or late, but I can´t hear Henkka instead.
In addition to the supporting, Henkka also always sings a few lines on his own where there's a lot of vocal action going on so Alexi can catch his breath. Vocal parts which are exclusively Henkka's in a live situation include AYDY chorus "through a mirror I behold" and in the Decadence chorus "We are children of rebellion" and "We are children of decadence". It's a shame that his mic isn't often loud enough to be able to hear it really well because he is doing a pretty good job!
Also on-topic, I guess it's very likely that they just covered the English version Anthrax did of the Trust original!
^^Well you could hear Henkka´s voice and backing vocals pretty clearly at the Graspop thing, that´s why I was pretty sure I could make out the said part. You can hear him supporting Alexi in Children Of Decadence quite well there, or did I see it on the vid? Anyway, I didn´t manage to find it now. Either you can´t hear Henkka at that part or - more likely - I wasn´t concentrated enough all three times I´ve listened to it now.