if you set up site 1 as extension range 200 - 299 and site 2 as 300 -399 and use the UC540/560 multisite then the default intersite dialing will be
Intersite access code + site code + Extension.
ie from site 1 to site 2 extension 301 you ould dial 82301
and from site 2 to site 1 extension 275 you would dial 81275.
to keep the 3 digit dial you can setup floating extensions in site 1 from 300-399 that do a forward to 82301-82399, you can even give them the same names as the names on the other system and vice versa on site 2
So from site 1 to dial site 2 extension 301 they ould just dial 301
this then is floating ext 301, which is a divert to 82301 and then the UC on site 2 handles the call as normal.
It does give a bit more management on the systems in having ro create each extension 2 and such but it works and we have used it at several customers and after all thats what juniors or trainees are for
Hope this helps