I go with NO.
Barb, Dwarf Defender, etc, with DR X/- is DR that can't be bypassed.
It negates it from "any source", it's special, it's nothing.
Is nothing = to +1? No.
Is nothing = to +4? No.
Is nothing = nothing? Yes, but NOTHING can bypass "nothing".
So, it can't be used to bypass someone elses DR regadless of what the DR is be it X/+1, be it X/+4, be it X/-.
So a Barb can't use his DR to bypass another Barb's DR ether.
