Sequential turbos

[derherr65]

I keep thinking that sequential turbos would be interesting.
Now to clarify sequential turbo means you have 2 turbos, one small, one big. The small one spools up fast giving you low rpm boost. The big one kicks in at mid range somewhere and usually puts out a little more pressure than the smaller turbo. Result being power from off idle to max revs. This is not compound. One does not feed the other.
Do I have the room? I don't know. Do I?
Do I have the budget? Not yet.
Do I have the skills? Probably. I might have to farm out the aluminium welding.
Do I have the needed intercooler or pump mods to make all this turbo stuff safe and worthwhile? No.(Boy you guys are hard on me!)
But to the theory discussion anyway. (all mechanical parts, I hate computers in cars)

General idea:
Exhaust manifold lets out into a exhaust cut out. Cut out is actuated by a boost solenoid. Closed routes exhaust to small turbo, open routes exhaust to both. Both turbos exhaust into a large exhaust pipe.
Filtered air is routed to both turbos, but there is a one way valve or flapper immediately before each turbo. Both turbo outlets are plumbed to the intake manifold.
So I'm thinking the little turbo should be something slightly smaller than the stock KKK, maybe a GT1544, run at about 10-12 PSI. The big turbo should be a little bigger than the stock T3, a GT2259 maybe run at 16-18PSI.
So you would have lots of low end and enough top end that you'd need a myna pump to overfuel it. I think.

What are the pitfalls?