I have data template dt1 in sitecore that has the field \"header\" in section \"data\". I also have data template dt2 that has the field \"header\" in section \"portal\" Finally
You can reference fields by their IDs:
C#:
string value = item["{00000000-0000-0000-000000000000}"]
or
Field field = item.Fields["{00000000-0000-0000-000000000000}"]
I haven't tried this, but I think it'll work in XSLT as well: