the problem i see is that there is no more any plausible deniybility for receiving payments. this can be important if any gov decides that they want backlist addresses.

but i like the idea! maybe just let the sending address decides how many keys (and which) are required to send those coins? for me its an essential cryptocoin feature to need more keys to send money (but companies can easily build this on top of a minimalistic coin - it isnt just that secure)