You might consider using a cryptographic protocol for secret sharing [1], such as the one of Shamir [2]. These protocols divide a secret into shares, so that the secret can be recovered, even in part, only by assembling a specified number M of the total N shares, M < N. You could give one or more shares to each of several custodians, as numerous and diverse as your budget and requirements dictate.

[1] https://en.wikipedia.org/wiki/Secret_sharing [2] http://dl.acm.org/citation.cfm?id=359176