regardless of how many turbos a toyota engine has the code will always have one T
nissan use two T's to denote twin turbos, and beacuse you see many references to the RB26DETT in magazines people pick up on it and incorporate it into other manufacturers codes
we have even witnessed someone refer to the 1JZ-GTE engine as a JZ25DETT
for more info on toyota engine codes see this link
as for the RA28 thing, there are 3 possible supra twin turbo engines that your friend could be referring to
1G-GTE
1JZ-GTE
2JZ-GTE
your "learndeded" friends need to be more specific, any of these engines will fit into an RA28 depending on budget, hell if the budget is big enough u can slot a 454 big block in there as well.