I can speak from experience - YES.....
Although I wouldn't necessarily say that its twice as expensive, there is considerably more cost to do a Twin Turbo build. The main reason that I found for this is nearly everything that you need is "custom" as there isn't a lot of aftermarket parts out there for TT builds. From the headers, turbo mounting, waste gates, fresh air plumbing, inter-cooler, oil supply to the oil return - everything needed to be custom fabricated or made.
One benefit that I have from doing this project is that I am now the proud owner of a MIG welder, TIG welder, vertical milling machine and a lathe - as I found that it was more cost effective to do the work myself.
I was aiming for HP as well as "looks" therefore I was determined to do a TT build. However if I had to do it all over again I would likely call Chris at Turbokraft and order a complete, large single turbo engine that incorporates proven / tested components.
Richard