Have you asked them why they are saying no? I understand wanting to help two piggies in need (as I am a frequent CL scanner for piggies needing a better home), but perhaps they are worried that the addition of 2 guinea pigs would be to much upkeep/making the place feel like a "zoo?"
I would look online to see if there is a guinea pig rescue near you, then find their facebook page, and post that CL ad on their page. Hopefully, someone at the rescue or who sees the ad will be in a better position to adopt them
Thank you! They've always told me they're happy how well I take care of the boys. I buy everything for them myself, but I've spent so much money on fleece stuff for them, I don't think they want to deal with me buying double the stuff.
Definitely have a calm sit down with your parents though and ask them why. Even if they don't change their minds, at least you will understand why; rather than feeling like they are being unfair or unreasonable