Maybe a different approach? Do these two extensions (256 and 288) need their own individual mailboxes? If not, you could make one of them the primary receiver of these transferred calls, put more SA buttons on it for places to receive them, and SSA them to the other extension.
If they both need to be independant, but receive these calls, you could create still a 3rd phone, renumbering IT to 398, and set up its buttons like this:
5-Blank SA-Ring-10
4-Blank SA-Ring-9
3-Blank SA-Ring-8
2-Blank SA-Ring-7
1-SA-Out SA-Ring-6
Then for 256 and 258, set them up as:
5-Blank SSA-398-10-10
4-Blank SSA-398-09-9
3-SA-Out SSA-398-08-8
4-SA-Voice SSA-398-07-7
5-SA-Ring SSA-398-06-6
These phones would receive their own calls on the SA buttons on the left column as normal, but the calls to 398 on the right column.
