I ran some tests. Before I go into the results, I spoke to a mechanical engineer, and his feeling was that as long as the surface area and energy was the same, the difference between 2 X 8 oz vs. 1 X 16 oz would be very minor. We'll see...
-Breezy with some strong gusts
-Air temp 90F
-Water temp 77F
-Snow Peak 600 w/homemade lid (for 16 oz), 3.5" dia., ~48 cu in.
-Cup from Snow Peak Solo set w/homemade lid (for 8 oz), 3.75" dia., ~25 cu in.
-Homemade windscreen, 3" high, about 1/2" away from pots.
-Sunnyside Denatured Alcohol
-Polder digital meat thermometer
-210F = "boil"
OK, here are the typical results:
16 oz water, 29 ml fuel (about a fl. ounce)
210F @ 9:56, no leftover fuel.
8 oz water, 29 ml fuel (about a fl. ounce)
210F @ 4:56, first 8 oz. (Pot was dumped into a Sea to Summit X-Mug, refilled, then placed back on stove--about 15 seconds.)
210F @ 9:41, second 8 oz., no leftover fuel.
When the second 8 oz boiled, the first 8 oz. had dropped down to 167F. After both were combined to make 16 oz (167F & 210F), the net temp was 185F.
It looks like gross boil time is is about the same. With a stove that gives full output for 10 minutes (the Decagon roars for about 4 minutes after priming, then settles down), maybe some time savings would be realized. (This also suggests that the Decagon is optimized to boil 16 oz at once and conserve fuel--sort of.) The first cup's boil time includes priming (when the pot is not on the stove) so actual boil time is only 4:56 - 1:34 = 3:22. But by the time the second pot is added, the stove is about to settle down, and the pot is no longer receiving the same amount of energy as the first, so actual boil time is 9:41 - (4:56 + 0:15) = 5:11. Perhaps if you only need 12 oz for your meal, you could boil 6 oz. then 6 oz and realize more of the higher output in the first 4 minutes.
Is 185F good enough to rehyrdate a meal? Is 8 oz of 210F water followed by another 8 oz of water 5 min later going to ruin the meal? Given the variances of my own field cooking I'd say this is acceptable especially if using a cozy. Besides, I don't like really hot coffee or scalding food.
Alcohol stoves vary. Fuels vary. Environmental conditions affecting alcohol stoves vary. Windscreens vary. I purposely chose to do these tests outdoors in a worst-case scenario. But most importantly, hikers' needs and wants vary, and admittedly, breaking a single cooking step into two to save a few ounces won't strike a chord with most.
The Decagon isn't the most highly regarded stove; it needs to be filled fully for each boil for best performance. From what I've seen, other stoves don't have this requirement (they'll just burn shorter with less fuel), which may skew the numbers more favorably.
In less windy conditions I can usually boil 16 oz. in my Snow Peak 600 mug using only 25 ml of fuel, but the wind was a real drain for these tests and I kept running out of fuel, thus the odd 29 ml amount. (I dispense fuel with a child's medicine dispenser that looks like a syringe, and is marked in 10 ml increments.) Also, if your windscreen is too short or you dawdle changing out pots of water, you may experience flame-out on the stove in windy conditions. You may (or have to!) experiment with your setup to see if this works for you.
You'll need a place to store the boiled water if not using it right away; I used a Sea to Summit X-Mug, which is a 2.5 oz. collapsible mug made from silicone. Mostly too heavy otherwise, it happens to fit perfectly as a cap to the Snow Peak Solo cup when collapsed, and holds 16 oz. of water. (It also provides a larger cup of coffee than the Solo Cup does.) Inside the Solo cup I can fit my pot lid, a folding spork, a tiny scrub pad, and a tiny bottle of Dr. Bronner's soap.
1.9 - Solo Cup + Homemade Lid (the lid by itself is immeasurable)
1.9 - Total
25 cu in volume
3.2 - Snow Peak 600 Mug + Homemade Lid
48 cu in volume
40% reduction in weight and near 50% reduction in volume. (OK, if you include the X-Mug, add another 2.5 oz., but the volume remains the same.)
So if you're looking to trim pack volume a bit, and you can use boiling water in 8 oz. increments, and you don't mind fiddling with a pot change, and you don't necessarily need boiling water for meals and coffee, then this may work for you. YMMV