Not at all what you'd want. They're still way too close in size to not impact performance. I'd also fear putting anything over 15psi of boost into these engines due to their very high compression ratio. Besides the differences needed in compressor and turbine wheel diameters (and flow characteristics). You'd also need to take into consideration the exhaust housing specs. A non-gated secondary (the stock C turbo) would require a external wastegate to spool a larger turbo properly.
To pull off a twin'ed multifuel. You'd need to plan a few things. If money weren't an object, I'd build one with a balanced bottom end, main girdle, peened rods, shaved piston bowls (to reduce CR), a LDS injection pump, ARP studs on the rods, mains and head bolts, O-ringed heads, a full P&P on the heads, re-route the coolant away from the intake manifold, one off exhaust header, a HX35 turbo with a HT3B primary.
That would probably make for a substantial powerplant that still retains the multifuel capacity. I doubt the transmission or clutch would hold up long behind it. But the cost wouldn't be worth it. You could easily swap in a 5.9 cummins and lay down the same power numbers for a fraction of the cost.
IMHO, get a boost and pyro gauge and turn the pump up a tad and be happy with what you've got.
