Mycelium Entropy can split a generated private key into three parts in such a way that:
any two can be combined to recover the key; and
no single part reveals any information about the private key that can assist an attacker.
This method is commonly known as Shamir’s 2-of-3 Secret Sharing Scheme. It is implemented according to the specification at github.com/cetuscetus/btctool/blob/bip/bip-xxxx.mediawiki. Funds from a split wallet can be spent with Mycelium Bitcoin Wallet. The private key can also be recovered from shares with this tool:mycelium.com/assets/entropy/checksalt/sss.html.
Mycelium Entropy does not currently support this scheme with Hierarchical Deterministic wallets.