This has been done before. Best way is to use 2 small units to spool the next 2 larger units. Main problem is routing twice as many support items: coolant lines, oil lines, return lines, air intakes, intercooler pipes, exhausts, etc.

I believe the Bugatti supercar has a quad-turboed engine...
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Old: 1989 Formula, 383", DFI, built 700R4, TFS heads, Mini-Ram, Vortech S-trim 15 psi, 3.70 gears
Best ET: 10.796 @ 125.8 mph
New: 1989 T/A, 406", Accel Pro-Ram, DFI, AFR heads, HRC Turbosystem, NOS 150-hp dry kit, TH400 w/GV overdrive, 9" w/3.70 gears, 8-pt. S&W rollbar, 18" wheels
Best ET: Spring 2001
Employed at:
Hahn Racecraft
Accel EMIC/DFI Tuning
Turbosystems & Custom EFI
Member of Midwest F-Body Association
www.mfba.org