No, of course not... anything is possible because it's fantasy. But is it a good idea? When you create a fantasy world, in order to have it read as plausible and allow the reader to suspend disbelief, you have to establish rules for your world and maintain consistency. When you pull something out of a hat like this for no reason, it damages suspension of disbelief because it is inconsistent when everything that has gone on before. They could choose to bend the rules- and say, well alien biology and all... but it would still be out of the blue and fully inconsistent- since this has never been shown to be the case before. And furthermore, the characters don't care and don't think it matters who the father is- and the whole point of that would be completely undermined if Sust did turn out to have two dads.