Tips
Try to reduce the contact time in between both kicks. The hook kick should be followed up fast.
Spot on both kicks and try to hit the same target twice.
Rotate your hip with the round kick, so you won’t need to spin much for the hook kick. You hip should face the direction of your spin, while you round kick.
You can try to do this trick slowly first to get a better understanding of it and then speed it up as you improve.