Stalled diet - ideas for a jump start?

Kerra was once over 22lbs. She was 4 years old at the time. She's been on a serious diet for a little over a year now, and she's stalled out loosing weight. She has to be on special food (Purina Rx UR) because of a reoccurring bladder problem, but our vet has given us a specific amount to feed her daily and it was working. We break her food up into two meals, so she eats every 12 hours. The problem seems to be that since she's lost weight (she's down to just over sixteen pounds now) she's discovered she's able to jump up to where we keep our other cat's food! Jack is not on a diet (hot metabolism breed) and he screams angrily when he discovers Kerra has eaten all his food. Kerra has stopped loosing weight, not to mention that she is not supposed to be eating any other food for medical reasons. How can I stop her from eating Jack's food without starving Jack?? I'm out of ideas! Wherever he can go, she can go too now.

Asked by Captain Jack on Aug 2nd 2010 Tagged diet, foodstealing, overweight in Food & Nutrition