Try using a fert that has a low first number and a high second number. If the only thing you can find is like a 15-30-15, try mixing it half as strong so you're actually giving them 7.5-15-7.5, so the foliage growth isn't as heavy. You could even dilute it more than that to discourage additional foliage growth.

One thing I've noticed about double impatiens is that since the blooms are quite a bit smaller than regular impatiens, it doesn't look like it's flowering as much, when in fact it's flowering quite well. It's just that when the blooms are bigger, they visually cover more of the plant.