Like everyone above said...depends on the device. On an ideal device, it's double vaping output using about double the amps at the proper voltage. However, that means the device has to approximately double the electrical output. Some can't. So it is "starved" and ends up splitting the max output, which may be less than the ideal.
The other question I have for you is...what voltage did you run it at? lol. You may have faked yourself out. Let's use a 1.5 ohm DCC as an example. If you run a normal 1.5 ohm coil at, say, 3.5 volts and you try to run a 1.5 ohm dual coil at 3.5 volts it won't work well. That's because the 1.5 ohm dual coil is really two 3.0 ohm coils. So you'd want to run them at 4.8 to 5.0 volts, because that's what you'd run a 3.0 ohm coil at. See?
I'm not sure that this is really accurate, voltage is voltage, but what varies is amperage and wattage. So if you have 2 coils, and are doing 10 watts, each coil (I believe) is only pushing 5 watts.
Where this gets complicated is that a single coil at 5 watts will produce a cooler and thinner vapor and you will likely get a less total vapor between 2 coils at 5 watts than one at 10watts as I believe 5 watts is a less than ideal output for optimal vapor production (this is when you would actually start getting into efficency regarding specific vapor points of your eliquid of choice, optimal temperatures, heating surface area, etc).
To compensate you increase the voltage to increase the wattage (which results in higher current draw which is where you might find some batteries, especially ego types to be lacking)
If my understanding is correct (and it may not be):
If you have a 2 ohm single coil and you want 8 watts of power, you would need 4v which would draw 2 amps.
If you wanted to double your vapor production, you would want 8 watts on each coil, which means 16 watts total, to get this with a 2ohm dual coil you would need 5.66v and would draw 2.83 amps.
I believe something like the ego twist has a current limit of 2.5 amps which would peak out at 12.5 watts or 5 volts. But, since a twist peaks at 4.8v, with a 2ohm dual coil your would peak out at 11.52 watts and 2.4amp draw. This would only be 5.76w per coil.
Now with a device like a vision, while you might have 2 coils that are fairly standard size, I would suspect that the wicking and am certain the airflow would be reduced from a single coil device, so while you could conceivably do 16 watts on the dual coil, you would likely end up with dry wicks and a burnt taste.
The other thing to keep in mind is that if you double vapor production at the same wattage (2 coils both at 8 watts), on an ideal device with adequate airflow and wicking, you are also doubling the total heat being output. And while this heat is displaced in twice as much vapor that also means twice as much surface area for thermal transfer, which I would suspect would give the impression of hotter vapor. To counteract, you would likely either take smaller and faster draws (which I suspect is the bigger benefit to most people), or lower the heat output by dropping the wattage, or you can do a combination of both until you find a balance.
So at the end of the day dual coils primary benefit is not so much the doubling of vapor production so much as being able to produce an equivalent volume and temperature of vapor faster. The end result is not so much a better experience as a different one.
So two coils at 5 watts (your example) means that the battery needs to supply 10 watts. If it can't voltage drops when it's amp starved.
