It's not much proof on my end but I knew a few people who had known both Peaches and SandyPants, and it was mentioned to me years ago that they were one and the same. I had forgotten about both accounts until this all came up. It may have been an open secret of sorts?
If you look at both of accounts and see who they drew art for, and who drew art for them, it's basically all the same people. The same names keep coming up between both accounts.
There's also this lamb picture on Peaches' account that looks very familiar to SandyPant's lamb character:
Like I said, not much proof on my end. I can only tell you what I've noticed and what others have told me, sorry.