You could do this with a shared line. For example:

(Portions of config omitted)

ephone-dn 1

number 1000

ephone-dn 2

number 1001

ephone-dn 3

number 1002

ephone 1

button 1:1 2:3

ephone 2

button 1:2 2:3

In this example, 1002 is a shared line on both phones. When you call 1002, both phones ring. If you don't want the directory number to appear on the phones, you could also configure the shared line as an overlay. For example:

ephone 1

button 1o1,3

ephone 2

button 1o2,3

Hope this helps. If so, please rate the post.

Brandon