Pet store salesmen are idiots. Do not trust anything they say. Ever. They are purely there to make a sale, and do not care a wit for the well-being of the fish they are trying to sell you. After all, the sooner you kill your fish, the sooner you'll be back in the store to buy more.
Betta fish are extremely territorial. This goes for both males and females. In fact, many females are even meaner than the males, and will beat the living daylights out of them, rip their fins off, and kill them. Bettas are generally best off alone, especially when it comes to other betta fish. Please remove one of them right now and put them in another tank (or if you don't have one, stick one of them back in the cup they came in for the night and either return one, or go get another tank as soon as you can). If you don't you are likely to wake up to a dead betta or maybe even two in the morning. The only time males and females interact with each other is for breeding purposes, and they have to be specially conditioned before you can do that, just throwing them in a tank together will not work. And even for breeding, the female must be removed immediately afterwards or things will turn ugly fast between the two fish.