Hello,
Create a line group with the 2 extensions that you need to ring. Create a hunt list, and add the line group that you created. Create a hunt pilot, and associate the hunt list that you created.
On the first extension, for call-forward no answer, forward to the hunt pilot that you created.
To forward the call to voicemail after the hunt list has been exhausted, just set the call forward no answer / call forward busy settings to forward where you need it to, or you can set it to use the personal preferences of the phone if you wish.
Hope that helped, if so please rate.