I don't know what is double track technically, but what was the thing I witnessed during the 2013 tour ? I was really surprised when in sofia in 2013 Jon left the stage (caught by some serious allergy I think though he was not coughing) and he let Dave sing In These Arms alone and all of sudden you hear extremely clearly Jon scream (not back vocals to me) - Dave was himself quite surprised by it - as it was during a time he was not singing himself and couldn't cover Jon's voice fast enough - he panicked a bit but not many noticed (I recorded the song in full )

Is it a normal practice ? I asked a few friends and they told me they all do it ....