How does cutting a pork shoulder affect cooking time?

As moscafj pointed out, the old minutes-per-unit-weight guides were flawed, largely due to the fact that the rate at which the center of a piece of meat heats up is more related to how far it is from the surface than the mass of the whole piece of meat, and those two factors depend a lot on the shape of the meat; consider a long, flat hanger steak cut from the diaphragm, relative to a much thicker piece of round roast from the back leg; it's easy to see that even if both weigh a pound, the long, flat one will cook through more quickly than the roughly-spherical chunk.

What moscafj's answer missed, however, is that pork shoulder is a hard-working piece of meat, responsible with holding the pig off the ground and as such, has a lot of connective tissue that takes time at temperature to break down. moscafj's advice would be very sensible and indeed outright correct for a relatively tender, fast-cooking cut like a loin or rib roast (though these are not often smoked, simply because a smoker isn't optimised for said fast cooking).

For a slow-cooking cut like shoulder, however, both the time it takes to hit minimum temperature and the time it spends afterwards breaking down collagen and becoming tender are important. One can approximate this by designating a higher temperature to finish cooking at, reasoning that the time it took to get from the minimum temperature to that one is about as long as it takes for the collagen to break down, but this is still equating a change in temperature with an amount of time, which can be unreliable as described in my first paragraph.

The thing to check for with a cut like shoulder is tenderness. Kenji Lopez-Alt's recipe for slow-roasted pork shoulder calls for an 8-12 pound bone-in, skin-on shoulder to roast at 250 degrees Fahrenheit/ 120 degrees Celsius 'until a knife or fork inserted into the side of the shoulder shows very little resistance when twisted, about 8 hours'. Using a smoker instead of an oven shouldn't affect that timing.

Regarding your actual bolded question

Apologies for spending three paragraphs pontificating; I wanted to make sure we had a solid grounding in the reasoning behind my answer, rather than just providing a recommendation.

Cutting your roast in half will reduce the time it takes to hit the minimum temperature to start breaking down collagen, but it won't affect how long the collagen itself takes to break down once it has hit that temperature. I would estimate that somewhere around five to six hours, assuming the 250F/120C cooking temperature, would be a good time to start testing for tenderness with a fork.

In terms of tips on how to physically split the shoulder, I imagine that a 10lb shoulder includes the bone, unless you're dealing with a very, very large pig, and such a bone presents challenges; the bones in a pork shoulder are large, run more or less through the entire cut, and are oddly shaped. Unless you've got a butcher's bandsaw, I'd recommend against trying to cut it in half yourself - though if you've not bought it yet, and you're buying from somewhere with an actual butcher's counter, there's a solid chance your butcher will cut it in half for you if you ask.

Cooking by minutes per pound is far less accurate than using a thermometer. The challenge with a pork shoulder is that it is made of of different muscles. So, timing could certainly depend on which you are cooking...and, when using the charcoal grill, obviously the cooker temperature is going to be a factor. So, get a good thermometer and rely on that. Use the shorter end of the time estimate for an indication of when to check the temperature.