How to program 2 users that share 1 phone

By design you have 1 person per phone so these are cheats or workarounds based on other options available.
You must use a spare port on the phone system for the 2nd person, it does not need a device or phone plugged in.
I.E. on a MICS with a 6 port exp card then any DN will do on that EXP card, not even the need for a 0X16 would be required.

1st person has their phone with their ext
1st person has their mailbox send message signal to display when message left
2nd person has their ext as a button on above phone
2nd person has their mailbox call them on the button when message is left.

-Lets assume the physical set DN/EXT is 200
-Pick an unused port on the system and get the DN # (lets pretend its 300)
-Go to Sets/200/LineAccess/AnswerDN and add 300 as Appear&Ring
-Go to Sets/200/Capabilities/ForwardNoAnswer/ to voice mails DN and 4 rings.
-Go to Sets/300/Capabilities/ForwardNoAnswer/ to voice mails DN and 3 rings.
If the 2nd user has private line*:
-Go to Sets/300/LineAcess/LineAssignment and add the line required
It is important that the ring count for Forward No Answer is per above where 200 is higher at 3 than 300's ring count at 4.
-Create and/or edit mailbox 300, for Ext: entry enter 300 so looks like Ext:300.
-Once mailbox is initialized go into ADMIN/8 then Admin and setup it to call Ext 300, the hours to be notified.
See this FAQ as well for more info on mailbox programming http://www.tek-tips.com/faqs.cfm?fid=3778

When a call come is it will look like XXX>300 (300 equals 2nd person) and it will go to mailbox 300 after 3 rings.
When a message is left for 300 then vmail will call 300, looks like 500>300 (500 = vmail and 300 equals 2nd person), when you answer the call you will be prompted to press 1 then enter your password.

If you have Caller ID you will see it when you answer the call otherwise press F811 to see it prior to answering since you would normally see something like XXX>300.

*
If the 2nd user has a private line but does not need the line to ring the set an is using Callpilot for voicemail then you can use the Express Messaging Line in Voice Mail admin programming and assign the line within the required mailbox so that all calls on the private line will go straight to the mailbox.