1 key = 256 bits
2 key = 512 bits
1 key crack = 2^255 tries for 100% certainty, 2^128 for 50% chance of solution
2 key crack = 2^511 tries for 100% certainty, 2^256 for 50% chance of solution
on average, not just twice as hard because you have to try every key with every other key, so it's 2^256 times as hard