If I understand correctly, you want to capture 'pageName' in Omniture by mapping two different UDOs to it (as some pages use one, other pages use the other).
Page 1 = site_page_name = 'great page';
Page 2 = page_name = 'another great page';
In both examples you want the 'site_page_name' UDO and the 'page_name' ODO to show in SiteCatalyst as 'pageName'.
Yes, this is possible:
Screenie : http://note.io/16uVmhS
Note: I have used 'site' and 'title' as my ODOs in this example but they could easily be 'site_page_name' and 'page_name' as above
Is this what you were looking for?
Apologies if I have misunderstood.