asterisk users - Apr 2006 - Help on multiple dialed sip-channels.

Hi.

I have the following problem.

A person with two phones is called like this:
Dial(SIP/user&SIP/user2,20,tT)

So this is the case. If the dialled person is already talking in one of
the phones, how do I send the caller to voicemail, or a specific
priority? 

If the person isn't busy they both should ring. Does anyone have a clue
on how to solve this?

And if both phones is unavailable I also need to send them to a specific
priority. 

Thanks in advanced. Any help would be GREATLY appreciated.  

Regards,
Arne Morten.