If you have the ability, save up money and buy or make a C&C cage yourself, and that in itself will show responsibility. Like Kipsie said, once you convince them to allow you to adopt one, a second one shouldn't be hard. My parents were objective at first at the idea of me adopting a second girl, but I told them she does need a friend, and she will end up being lonely if she's by herself. Explain that guinea pigs typically (meaning almost always) live with other guinea pigs and become bonded together. Having two pigs usually makes for happier pigs and you'll see them running around together, cuddling together, and just enjoying each other's company. Tell your parents any of those things, and maybe show a cute picture or two, and watch their hearts melt and go awww!
Making the C&C cage is super easy, and if you don't want to buy a kit from here:
Buy a C&C cage, then you can do a search on this page:
Where? and find a store near you to buy coroplast at and cubes as well. Cubes are sold at Bed, Bath and Beyond, Linens n Things, Target, etc. You can ask people on here who are in your area where they found theirs.