All you can do is to do your research and present it to your parents. Guinea pigs really are better off in pairs or groups, and there is plenty of research out there to back that up. I'm not sure how old you are and whether you are working part time and so on, but you could also present ideas to help fund another pig, a larger enclosure (if needed), etc. Really, having two guinea pigs is not much different from having one. I can say as a mom, I would be more inclined to listen to my kids on an issue like this if they came to me with some research and with some practical ideas to make it work and some willingness to make some sacrifices (like putting in some money or offering to pick up additional chores, etc) and then followed through on that. Good luck!