We have a woman who teaches a knit maxi skirt class on a regular basis, and they hem it with a double needle. The trick is to adjust the needle bar position a little to one side or the other (don't know if your Elnita has that capability). Try both directions and see what happens. Adjusting the needle bar position, actually changes the timing just enough to make the double needle work.
It's hard to find a ball point double needle. You'll be alright with a 'universal."
Also, seperate the two threads by putting one in the lowest thread guide, and leaving the other out. That way, they don't twist together before they get to the needle.