There is a unique 2.0 litre Subaru horizontally opposed boxer engine found in the ~1994-1998 (Generation 2) and ~1998-2003 (Generation 3) Subaru Legacy (Subaru Liberty in Australia) line-up.
The performance models GT,RS,GT-B,RS-B,B4 and numerous special-editions were equiped with an extremely-complicated but rewarding Sequential Twin-Turbo system – what’s that, a 2.0 litre 4-cylinder Twin-Turbo!? This is not really your typical TT implementation.
An introduction quoting Subaru’s Technical Specification:
“The purpose of a turbocharger system is to produce a higher engine output while maintaining the same engine capacity. This results in an increased power to weight ratio and therefore improved vehicle performance and handling dynamics due to a better vehicle weight distribution. The turbocharger harnesses unused exhaust gas energy, to compress the intake air. Compressing the intake air allows a greater mass of air to enter the engine for each firing cycle thereby allowing for a corresponding increase in the amount of fuel injected. The result is a considerably higher power output depending on the ‘boost pressure’ without increase in engine size or weight.”
Fair enough, that’s the basics of any turbo-charged car -
“The problem however with a GT touring car using a single turbocharger is that the torque output is not evenly spread over a wide engine speed range, resulting in a ‘peaky’ acceleration performance. This is due to the delay in bringing the turbine up to its peak operating speed range when it is most effective in compressing the intake air. This is known as ‘turbo lag’. A small modern turbocharger has very little ‘turbo lag’ because of its low inertia and is most efficient at low to medium engine speeds. Its efficiency however is reduced at high engine speeds due to restriction of the exhaust gas flow and smaller compressor. A large turbocharger works efficiently at high engine speeds producing a high top end power output but at low engine speeds it suffers from excessive turbo lag caused by the delay in achieving high enough compressor speed to compress the intake air. By providing two turbochargers that work sequentially, the advantages of both turbochargers can be realised.”
In theory, yes! And this is where an obsession begins 
For those that know my ‘TT tinkerings’ on RSLibertyClub, OzLiberty and Perth-WRX – you’ll know I’ve amassed and always shared a some-what useful collection of TT documentation and photos. This will hopefully be a more organised and informational way for me to share the docs and maybe some of my tinkerings. I’m far from being a qualified mechanic (I’m a Software Developer after-all) but I’m hands-on and most of the tinkerings I’ve carried out myself or researched to death. I suggest if your planning anything on the EJ20TT that you do both of these; you’ll learn the most doing things yourself and research what your doing – taking everything with a grain of salt (especially forum banter, and probably anything I say).
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