Well, for one, it's called compound charging, not twin turbo and the plumbing is really complicated. I highly doubt you would want to go down that route as I don't think you have the engine bay space to accommodate it.
Advantages is that you get the power low down from the small turbo and also power up top from the large turbo, but the power up top in theory would be slightly lesser than if you were running a single large turbo only as the air has to pass through the small turbo which causes a slight restriction, but basically you get best of both worlds. Disadvantages is plumbing nightmare, need to buy 2 wastegates and possibly tuning nightmare as well.
Here's an image I borrowed off Google
So basically plumbing wise, exhaust manifold goes to small turbo, the output of that goes to the large turbo, the wastegate of the small turbo also goes to the large turbo, then the large turbo exhaust goes out as normal. For the cold side, the air goes through the large turbo first, then into the small turbo, then to the intercooler then engine.
At lower RPM, the exhaust gas is enough to spool the smaller turbo to reasonable speed, the compressor will then draw air THROUGH the large turbo and compress it, then once the exhaust gas is enough to spool the large turbo, the large turbo will push more air than the small turbo, and is basically forcing air into and through the small turbo. Basically you should be able to hear your turbo spool up twice, once at low rpm and another time at a higher RPM.
I also considered of doing this, but limited engine bay space and high cost and effort prevents me from doing so. Now I just run a single TD05 twin scroll turbo on my 1.8L engine and it gives me full boost from around 3.5-4k rpm all the way to my 8400rpm redline.