You'll surely get some answers more experienced in your regions waters, but I'll give you this one for starters...
You're talking about two piece Farmer John's, right? One piece looks a lot like what's common thought of as farmers coveralls :fork: (although I've been in a farm family for 50 years and have sledom seen farmers wear these), and the next peice goes over your torso next with long sleeves. Yes, the two layers of 3.5mm should give you a total of 7mm, with two exceptions:
(1) Not on the shoulders - where I get cold first, in air or water; and
(2) Water seeping between the two layers will make this not the same as a 7mm.
I seldom dive in water below 70 degrees F, so I'm not your best source, but still - I'll offer what I can.
(a) At 80 F and up, I wear a Polyfin;
(b) From 68 to 80, I usually wear the polyfin and a 3mm Jumpsuit - a once piece that covers my torso, arms and legs, maybe with a beanie, especially in the lower temps of that range; and
(c) Below 65, I'm wearing my polyfin, my 3mm jumpsuit plus a 3mm shorty - giving me 7mm over my torso including my shoulders plus a neoprene beanie. :cold: I was cold at 55 F last week at Catalina, but part of that is that I'm not acclimated for such waters.
I may be wrong, but I like my approach, with two layers on my shoulders.
Someone else.....?
DON
:mean: