You don't want to cut the key in half. There are lots of complicated ways to share secrets (some even involving redundancy i.e 3 of 5 key fragments needed to rebuild key) but the simplest way is an XOR.
http://en.wikipedia.org/wiki/Secret_sharingI will use 16 bit key as an example.
your private key: 1100010111010011
key partA: 0011101011010011
key partB: 1111111100000000
If you XOR (exclusive or) the two key parts you get the full key.
If you have an existing private key you can generate A randomly and then perform an XOR between full key & A to produce B.
Even with their key part the keyholder still can't brute force with any reduced difficulty. Collusion is always a possibility but you haven't reduced key strength.