[helene schoubye]

This indicates different species. Females with longer ventrals than males are completely unknown. Anyway, the ventrals are mosly a clear indication of species identity.
So, you certainly have bought at least two different speceis. I hesitate to renew my suggestion that you should breed them. Better have new look into the tanks of the trader, speak with him, and try to get another female fitting to the male’s ventrals and another male fitting to the female’s.
And we can learn from this: The description can in most cases be more exact than one thinks is sufficient in the first run. And I was too quick with answers, too.
And: Try to make photos!