*SMOOCHES* to those two cutie pies!
Your problem is one that happens often in a multi cat household. It's hard to give one what they need and keep it away from the other. If it were me I would do more one on one interactive play with Mr. Lazy Bones. They are still so young that I would hate to start diet food to early especially with one being so active.
Get yourself a laser light, sit back on the couch, and have a ball!! You can really get some serious activity with that!