A large rat every 2-3 weeks is plenty of food for a boa that size. Offering a 2nd prey item on occasion won’t hurt anything but I wouldn’t do it consistently. You’re not really feeding for growth anymore at this point, you just want to maintain a healthy body condition. With the exception of my colubrids I feed all my snakes every 2-4 weeks after their 2nd year and only once every 4-6 weeks during the winter.