basically, you need the base turbo of your choice, i.e k03s or k04-001, then your bigger turbo you want to hybrid with like the k04-023.
you`ll also need a rebuild kit, preferably with 360 degree upgrade bearing.
then its a case of mating the hotside of your base turbo of choice to the cold side of your bigger turbo.
the smaller the hotside, the faster it will spin the cold side. but the smaller the hotside the more restrictive it becomes.
when it comes to mating the new turbo you could either keep the new housing for the build (this would mean you would need the TIP and associated boost hoses from the doner turbo) or you can machine out the housing of the base turbo, this is the route i would like to go but means finding someone or a way of enlarging the housings port.. i was wondering if a brake caliper honing tool would be man enough for the job.