Well the thing is that even in supras, when they want to reach 900 bhp+ they remove the twins and install one bigger and lighter VNT. Twin turbos sound awsome but i don't think they would be any useful on tha already straggling engine of an R5.
If u actually wanted this to work, i guess u would need 2 T2s each each installed in 2 ony exhaust pipes, plus superlightweight units in terms of fans ofc.