Could be male and female.
From experience, let me tell you that generally shop owners know very very little. I've talked to owners who swear that dart frogs are their "specialty" and as we've talked I've realized that they know less than I did. I'd be very skeptical about ANYTHING that they tell you. It seems like half the time they'll make something up when they're asked a question, instead of saying "I'm not sure".To be honest, I'm not entirely sure of their age. The owner of the shop (the one with the knowledge but not the breeder) wasn't available.
IMHO I would separate them until the small one puts on a bit of weight. It does look to be a pair and I have had some pairs where the male will not eat as much with a female present, polite I guess But I would I would separate them for a little while (month or two) then put them back together.