asterisk users - Jun 2008 - capture call within same callgroup with *8

I would like extensions within a specific group to capture calls (with the
typical *8).

All's fine if the extensions are registered to the same asterisk server.

However, I don't know how to make it work if they register to different
Asterisk servers within the same LAN.

I use DUNDi to call from an extension on one box to another extension on another
box (or simply DIAL a IAX2 friend trunk between the boxes).

How would I go about "capturing" a call ringing on ext1 from ext2 with
*8 or similar knowing that ext1 is registered to box1 and ext2 to box2?

Thanks,

Vieri