Ids will grow very large given the chance. When I was a kid I had 3 in a 20g for a few months and their growth was minimal. As an adult I bought two 1.5" ids and they grew to 26" in less than 2 yrs in a 750g tank. They are a good example of a fish that will stunt easily if not given proper conditions.
I'm assuming you bought them from different places? heres another reason to think of it. if you put a rtc, tsn, or any of those larger cats in a 20 gallon tank they will still grow large and will soon not be able to turn around. there used to be a vid on youtube of a 40" rtc in a 48" 75 gallon tank. some ID sharks will do this(check out the link i posted) and some won't(see link). this is just the way I see it. they very so by size so much that it amazes me.