From what I remember from school, Rachel a had a sister called Leah, and Jacob wanted to marry one of them but because it was the younger one he loved, their father would not let him and tricked him into marrying the elder (she had a veil on so he didn't recognise her and I think this is where the veil tradition at weddings come from). Their father finally agreed that Jacon could marry the younger one aswell, but I think he had to work on his land to 'earn' her. So he ends up being married to both. Now correct me if I am wrong but he would not impregnate the one he did not really love (there is a bit about spilling seed). However one of his wives did have children because they were the twins Esau and thingummy and they turn up in the parable of the son returning and Jacob killing the fatted calf.
Really sketchy, but quite impressive that I remember it at all!! Quite near the beginning of old testament so easyish to find.