asterisk users - Feb 2004 - Call Groups and outgoing line selection

I have 2 lines setup. One is the house line, the other a business line.

What I'd LIKE to do, is if a house extension dials out, it selects the
house line to dial out on, but if the house line is busy use the
business line.

Ditto with the office extension, but reverse. 

Using distinctive ring on both lines, I can do a CO forward on busy to
the alternate number for each line, and maintain correct context
depending on which number people called.

My guess on how to implement this is to duplicate dial plans for outside
numbers, and use macros to dial out on each prime channel, and use the
alternate if congestion.

This make sense? Anyone do something like this already and have an
example?

I've looked through a lot of different pieces of documentation regarding
*'s ACD functionality.  Is there any one place in particular with a good
amount of documentation on it?

Thanks

Mark