Yes, your beardie sounds a bit on the small side. When I got my beardie, I was told he was 4 months old, and he was 13 inches long. I know there are several members of this board who have 4 month old beardies that are upwards of 16 inches :shock: and eat more than 100 crickets a day. There is a beardie named Toothless who was almost 19 inches before he was five months old (viewtopic.php?f=49&t=173042)! But no two beardies are exactly the same. As long as he's eating and growing, and you have the proper UVB light, etc, then you shouldn't have to worry. It's a good idea to regularly weigh and measure him and keep track in a notebook, so that you can tell how well he's growing. I actually keep track of everything from how much my beardie eats, to how often he poops, gets a bath, etc. If your beardie isn't growing well, you may want to have a fecal exam done to make sure he doesn't have parasites. 100+ crickets in two days sounds like he has a pretty good appetite, so if he's not growing, it may be because of parasites. Also, I've always heard that at that age they should have 1 feeding dusted per day with calcium 5 days a week, and multivitamin the other 2 days.
And for your original question, I agree with everyone else that it is definitely not worth the risks to house the two of them together.
At the pet store closest to my house, the product manager has beardies at this house that he keeps together. All adults, two females, two males (I think) and he said he has never had any problems what so ever.
On the other hand, the beardie breeder at the reptile show strongly advised against it.
I think it all depends on the personality of your beardies in the end. Just make sure you have a second enclosure just in case.
Two males tend to compete for territory.
A male and female have a chance of over breeding and the stress can kill the female.
Two females is said to be your best bet but still have a chance of being territorial.