I have a lot of experience with parrotlets since I have been a hobby breeder of them for about 7 years now. Anytime a customers asks my opinion on this I always tell them to just get one! Parrotlets are still a big commitment and require just as much care and attention as large parrots.
If you want a good pet parrotlet you should just get the one. Just because they are small doesn't mean they are easy or don't need what the big parrots need!
With either two males or two females you always have the bird that is more dominant and will push the other around. Which is not fair to the bird that is always being bossed around. Also as others have mentioned they are very territorial and this can lead to bad fights and injured or even birds being killed.
With the male female pair even if you don't have a nestbox there can be aggression when they are sexually mature. The male may want to mate, but the female isn't interested, and you have a male that will now bully and pick at the female.
It's best just to get one as a pet, but if you want more than one I think it is a MUST that they be housed separately. Not sure why the breeder would suggest getting two at the same time, and a male and female that will likely only want to bond with each other. I really hope they aren't siblings.
Also, be sure to give them each one on one time so they aren't bonding with each other instead of with you.