[asterisk-users] Can one SIP extension be used for two phones?

[alunt2003 at googlemail.com]

Picture this:

Exten => 100 #My Phone
Exten => 200 #MythPhone

Call comes in. Dialplan calls both extensions.
MythPhone is an add-on for MythTV,so when i receive a call,the CallerID
is flashed up on my TV.
I want to add another MythPhone to my other MythTV box upstairs.

Do i have to make a third extension and get the dialplan to call all
three extensions or can the second MythPhone instance also log into
Asterisk using the same extension?

Thanks, Alun