Is it better to use one nail or two when installing hemlock bevel siding?

Hemlock is a beautiful wood but it's notorious for moving, and your foreman is 100% correct on this one. When you're installing wood lap siding, the golden rule is to allow for natural expansion and contraction. By nailing a single 6d or 8d siding nail just above the lap of the previous board, you're securing the piece while letting the top edge slide slightly behind the board above it as the moisture content changes. If you drive a second nail higher up, you've essentially pinned the board in two spots. When that hemlock tries to shrink in the summer heat, it can't move, so it just splits right down the middle.

The new guy's concern about cupping isn't entirely baseless, but his solution is wrong for this material. Cupping usally happens because of uneven moisture absorption or poor back-priming, not because of a lack of fasteners. If you're worried about the boards pulling away, the fix is usually narrower exposures or better wood selection, not doubling up on nails. I've seen plenty of houses where someone thought they were being 'extra secure' with double nailing, and two years later every single board had a foot-long check starting at the nail site.

Meeting in the middle of a wall while nailing from opposite ends is allready a recipe for a wavy disaster, even if they were using the same pattern. But doing it with two diferent fastening methods is just asking for the boards to bind and buckle. You're basically fighting the tension of the wood from both sides.

Check the manufacturer specs if you need backup, but almost every siding manual for hemlock or cedar specifies that single-nailing is the industry standard to prevent splitting. Since the foreman is the one who has to warrant the work, his method is the law on that site. Just keep your head down tomorrow and keep the cuts tight; hopefully, the new guy realizes he's lucky he didn't get fired on the spot for trying to override the lead's specs.

I did my barn in hemlock a few seasons back and tried double nailing on the first few courses becuase I didn't trust the single-nail advice. Big mistake. Those are the only boards that checked and split once the weather turned. I switched to one 2-inch stainless ring-shank just above the lap for the rest of the build, and it's held up perfectly. If you're worried about cupping, back-priming the boards before they go up does way more than an extra nail ever will. Also, make sure they're using stainless fasteners—hemlock reacts with galvanized and leaves nasty black streaks everywhere. That's a headache you don't want to deal with later.

Have you ruled out the moisture content of the boards as they were being delivered? I've seen guys argue over one nail versus two while ignoring that the hemlock was sitting in a puddle for three days before install. If the wood is allready at its max expansion, even a single-nailing pattern won't save you from some wierd movement once it dries out. Also, is the foreman checking the stud layout before they start from both ends? Meeting in the middle seems like a disaster waiting to happen regardless of the nailing pattern if the layout isn't dead-on. Could the buckling just be a stacking error rather than a fastener issue?