Place the lentils, split peas, onion, carrots, garlic, and cumin into a pressure cooker, and stir in the chicken broth. Seal the cooker, bring to pressure over medium heat, and reduce heat to keep the pressure stable; cook under pressure for 30 minutes. Remove from heat, and allow pressure to subside. When the cooker is at normal pressure, remove the cover, season with salt and pepper and stir in lemon juice to serve.