The places/people I know that will make twins are:Entrprise Engine, Piers, Performance Diesel, Quadzilla, Mark Kendrick (I think it's Texas Diesel Performance), and I think Scheids builds them as well. I'm pretty sure there are more but these are the places I've read about or talked to them about twins.
The cost varies with the turbos and wastegates used. The cheapest is using the stock turbo and adding a larger (primary) turbo, like a HT3B or BHT3B. For an actual cost of a system I would contact a few of the above mentioned places for a price.
BTW for an auto truck the heat exchanger has to go. In most cases you need to put in another form of transmission cooler therefore it is another expense above the cost of the twins.