press the combine key once and make sure the gems you want to combine are at the right side in that "storage thing" then just draw one of the gems onto the other and voila they are combined
http://www.youtube.com/watch?v=suGX9rAexB8&feature=related
I have rebuilt this maze and it worked well for the 15 points badge in medium difficulty
juggling means having two ways in the maze and then selling one tower at one end and block the other end so that the creeps have to return to the tower you just sold.