There are three major assumptions - firstly, that one is male, and the other female, secondly, that they are both fertile, and thirdly, they are young enough to reproduce several times.
I would worry about inbreeding, and diseases of genetic origin becoming prevalent.
Yes, it is possible in the condition that they are of different sexes, fertile, have an unlimited life expectancy, unlimited sexual relations as everyone can copulate with another freely, with uncircumcised minds about the myth of overpopulation, and other favorable situations remain normal. Females' sex- ration should also be higher than of males.