I do not believe that boost pressure is additive. Two turbos, each putting out 6 psi will just give 6 psi and twice the volume of a single 6 psi turbo.
The only value I can see to twin turbos might be an easier installation--one small turbo at each exhaust header. They would spin up faster than a larger, single unit, so you might get better throttle response. But quick throttle response in a diesel is not usually required.
If you fabicate up some plumbing for this, I'd like to see it.