A 1.5 ohm dual coil doesn't produce as much heat as a 1.5 ohm single coil. I realize that the resistance is the same, but there is still physically TWO 3 ohm coils on the dual coil. The same voltage isn't going to heat up both 3 ohm coils the same as is does the single 1.5 ohm coil. Dual coils have more vapor/surface area, but at the same resistance, when comparing the single and dual coils, the head of the element are not reaching the same temperatures. You're delivering X watts to one resistance versus two. When you deliver X amount of watts to a single coil, it ALL goes there at 1.5. When you deliver X amount of watts to a dual coil, yes, they receive the same power, but remember they are both 3 ohm coils, so they will each get less hot than the single coil.
Many websites have even said that it burns cooler, and also many run dual coils at higher volts because the coils can take it.
For those who like to read techy stuff.....
Series vs Parallel Electrical Connections