As I see it, the only reason to choke the un-turbined bank would be to match the thermal loadings across the the engine, however, one of the main reasons for only connecting one bank to the turbine would likely be space and weight.
Spool rate is only of importance in transient applications; since the turbine would need to be brought up from idle. In race applications like this, the turbine's own momentum and the consistently high exhaust velocity should prevent any problems. Once on boost the turbine should run very well "free", all the way to the wastegate opening.