Do you mean sequential turbochargers, or compound turbochargers?
Sequential is small turbo for low revs, big turbo for higher revs. (Like many 90s Japanese performance cars, Toyota Supra, Subaru Legacy GTB, Mazda RX7, etc)
Compound turbo is small turbo all the time, and big turbo feeding the samll turbo at higher revs.
Generally compound turbos are reserved for diesel engines because they make lots of heat. Pretty common on HGVs, Isuzu did a twi turbo D-max, which Id imagine would be compound turbo, Saabs 1.9 twin turbo diesels might have been compound, VW do/did it in some vans, Amaroks, and a Passat, and possibly some BMWs?
Theres a simple guide to compound turbo sizing - you pick the size of the small turbo by the RPM you want it to start making boost, and then the big turbo is sized by matching the outlet of the cold side to the inlet of the cold side of the small turbo.
As for the order of the air, it goes air filter -> big turbo cold side -> small turbo cold side -> engine -> small turbo hot side -> big turbo hot side -> Gretas face
Essentially whats happening here is the small turbo doubles whatever pressure is fed to it, so at low revs, when the big turbo isnt doing anything, the small turbo might double the atmostpheric pressure in the inlet manifold, giving you a bar/14.5ps/20kpa of boost. Then when the big turbo comes on, that might also make a bar/14.5psi/20kpa of boost, but now the small turbo is fed a bar/14.5psi/20kpa of boost, so doubles it, and now your engine is fed 40kpa, or 4 times more pressure than atmostphere.
Turbochargers arent perfectly efficient, so some of the work done to squashing the air is wasted by heating the air up, rather than increasing its pressure. This is why compound turbocharging isnt really done on petrol engines, because they cant handle the heat.
A TD05 is quite a big turbo, thats what you get on STI Subarus, and they have 2.5L engines which rev nearly twice as high as a 300tdi (and so flow twice as much air)
Just one more thing - you probably will blow up your engine doing this, unless youre very conservative with the boost pressure (in which case, whats the point)
To reduce the risk of it blowing up, Id advise you get a twin port inlet manifold (so all the cylinders get the same amount of air, rather than cyl 3 getting more than the rest, which is what the standard one does iirc, and its why cyl 3 is amlost always the one where the headgasket fails). Then Id fit the biggest intercooler and radiator you can to keep the temps down, and make sure the air isnt obstructed by numberplates, winches, mud, etc.
And exhaust gas temp gauge is a good way to get a feel of whats going on in your engine, what temp you shouldnt exceed depends on where you put the sensor etc, so I cant advise (and I dont know).
