I am not aware of any two-wheeled trailer with (intentional) camber. Are there any? If not, why don't two-wheeled trailers use a little bit of negative camber?
I think it would reduce the stress on the wheels in turns and also slightly improve stability (due to wider base). A small enough amount would not affect total width nor usable area as the wheels are widest at the hub.
Race cars used to use camber, sport wheelchairs use it (I think regular ones don't only because it would make them wider), so why not trailers?
I am willing to believe it causes an increase in rolling resistance, but I don't think it is significant enough.
I've been thinking about this for a few weeks now and I still can't see a reason why it would not work.