Is this time-dependent contract possible on the present protocol?

[Timo Y]

There is Alice, the main signatory. Then there is Bob, the secondary signatory.

The main signatory can spend the bitcoins at any time, with her signature alone.

But if Bob wants to spend the bitcoins, he needs to make two signatures. The second signature needs to be at least 6 months after the first.
 
Alice continues being able to spend the bitcoins, even after Bob has made the first signature.


My question: Can this be done?

My reason for asking is that I think this would be a great way to handle inheritance without having to trust a third party (eg. a notary or lawyer). If it can be done, I might be willing to pay somebody to code it, depending on how complex it is.