I have not been keeping snakes for very long myself... but one of my oldest corns, who is probably nearing a year in age, is at least 22inches long. How much are you feeding your snakes and how often?
And separate vivs are really really important. Stress can put them off their food, and slow their growth. I know this because it happened to me. I housed two of my snakes together for just a couple months, but Lia ended up twice Feren's size because Feren was stressed. Now that she's on her own, she's growing again and eating more regularly.