Just using the same encryption algorithm twice with two different keys of size N does not increase
the exhaustive search time from 2^n to 2^2n, thanks to meet-in-the-middle attacks, which reduce
the time to 4^n. IOW in return for doubling your key size you've increased the search time by a
factor of 2: That doesn't seem a good tradeoff.
Leave designing encryption algorithms to the experts: Personally, I know just enough to know that I
don't know anything like enough to start designing my own encryption schemes.