Giacomo,
There are a few different ways to skin this cat, but you absolutely have to a unique extension for each phone.
Typically, you would setup a Hunt Group and put both phones in an extension list and put that extension list in the Hunt Group.
Then you just give everyone the extension to the Hunt Group to call those extensions.
Don't forget to setup a mailbox for the Hunt Group and then set the Recall for the Hunt Group to be the Voice Mail application. The Recall Timer within the Hunt Group should be set to however many rings they want before sending the call to VM; every 6 seconds is one ring cycle so 4 rings is 24 seconds.
Then set the Hunt Group mailbox to light one of the two phones; unfortunately you can only light one phones Message Indicator unless you have Phone Manager.
Another way is to setup Dynamic Extension if the system has it available.
Thanks,
TE