I read that some dudes in australia took an original RB25DET motor and took the twin turbo setup from an RB26 and bolted it up. Are these motors that interchangeable? Like manifolds match up, internals dont need to be modded to take the twin setup, etc.? Sorry if this is a noob question.

Probably could be done with a lot of mucking around but there are far more cost effective solutions that will produce better results. Check out stage 2 in the R33 GTS25T engine tuning guide on the website in my sig.