Double? Just add one bit to the representation and limits of each.
You should be able to find those lines in Bitcoin Core.
I'm pretty sure that is not how it works. The secp256k1 curve is only for 256 bit private keys. To double the range, you be using 257 bit private keys and that would require an entirely different curve.
n = 1 to ( FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364140 / 2 )