Separate shipping from billing addresses

I need to separate the billing addresses from the shipping addresses at checkout. People should be able to add addresses at checkout (or in their address books), but the addresses should not be mixed up. Right now I can select billing addresses in the shipping addresses list an vice versa.

Normally this wouldn't be a big problem, but my client requests the addresses are splitted, and the addresses for shipping are limited by another extension I made.

The code does not allow me to modify this right now unless I wrote my own checkout module and address book module. But that doesn't seem like a good solution...

Is there any way I missed to achieve this?

What I did for the time being is adding the city name to the options array inside the checkout module, and inside the addCustomAddress hook I check the city name and remove all addresses that are invalid...

I think that the addCustomAddress Hook is your best bet at this point. We added it in there precisely for those reasons... Not sure there is any other way around it.

I used the hook to achieve what I mentioned at the end of my post. To get what I want I would need to make the addresses store what type they are, but also do this with the address book module. Should this not become a standard feature? I'm not sure why I would define two different addresses as a customer at checkout and want to mix that up later on.