Depends upon what kind of knees strikes. But a basic drill can be done with a stutter step involved and a partner.
Partner has two Thai pads, either crossed or one on top the other while crouched over. You, the kicker, grab around the neck of the pad older and knee kick into the pads changing knees and using the stutter step to bring the kicking leg down and then change the hip placement to throw the next leg into the pads.
You are trying to establish two things here: Flow and impact.
You can change the angle of the of your grab on your partner and just work one leg from his side as well. It's somewhat more difficult to practice on the standing bag, since the angle for impact is awkward. But you can practice penetrating forward knee kicks, or round knee kicks.