We have a 7 month old male and recently introduced a 3 month old male. The older one trills through the door, while 3M purrs when he sees 7M. They have met and will play fight, but then it always goes too far. 7M will stop and walk away but 3M will always pounce and start playing again. This pushes 7M and he then takes it too far. There's never any hissing or growling. But 3M does end up meowing and trying to get free. When I separate, 3M immediately goes back to playing. What do I do, should I just let them both keep going until they tire themselves out?