tesla bfilar pancake coil as a joule thief

I think this is really cool - it's a tiny little bifilar pancake coil - there are only 12 turns - being run in a joule thief configuration - i have no idea what kind of efficiency this thing has as i was pretty excited that it worked and shared it before i did any measurements - i'm going to play around with it a bit - as we do know it is certainly resonating - i wonder what will happen if i couple it with another pancake coil?