Few people here talk about real shoulders. What they usually get is a two pack of two pork butts. Have you opened the package and it's a full shoulder (you didn't say where you are from, but if it's not the south, it's likely just two PB's)
The 160 has a better heating element, so you won't typically see the 2 hours per pd.
But sounds like you haven't really used it much, so we don't know if the element is accurate.
Test the smoker to determine if the temp set is the same as the internal temp (just use an external thermometer to verify).
When you cut in in 1/2, then you'll have a picnic and a butt and depending on how it cuts you may have the picnic finish a little ahead.
I would start it the night before, AFTER you make sure it's working perfectly (don't do your first cook overnight on a new smoker, if it fails to work they you'll blame me)
If it was me, I'd test it, validate the temp and cook one of the two pieces first, then the other and just keep practicing.