That interpretation seems a little weird to me. I mean, how are you going to explain it to the kids?
"Right, so we've got this new Burny-Shock move here. It's a Fire and Electric type move."
*Kid butts in* "So it does Fire and Electric damage all at once?!"
"Er, well, no. It only does Fire damage. But Electric types can use it as if they were Fire type."
"But they can use Fire attacks just fine, see? He knows Flame Charge!"
"Yes, but he doesn't have STAB."
"What's Stab? Is it Steel type?"
It just seems like it'd be confusing and kind of a letdown that it doesn't actually do types of damage. The 'two attacks hit, each of a different type' idea seems best to me. I mean, kids aren't gonna bother to calculate out the exact effectiveness. They'd just have fun shooting lightning bolts that then set Pokemon on Fire.
And of course, there could always be no Dual-type moves.