Hiya Lynn,
You can already vape at 6V with 2 x 3V batteries- it's just you are likely to kill your atties a lot quicker, because they aren't designed for it.
Imeo has had these atties specifically made for High voltage vaping.
The 7.2+ high voltage atomiser is a completely new concept, previously if you tried to vape with 2 x 3.6V or 2 x 3.7V batteries, you would have popped your atty immediately. Imeo has fixed this problem with the higher of the HV atties (as you saw in his video where he nearly set himself and his house on fire

)
Of course, it's worth experimenting with these new atties on different voltages to see what results you get.