I can see it working all that well, yes it may halve the turbine housing, but it also halves the area the exhaust gas can use to turn the turbine wheel, so instead of the whole turbine wheel being used, only half is.
When the valve opens the resulting pressure drop would cause a big dip in the power curve, ala TT supra or Liberty B4
Mazda tried it on the S4 RX7 and then Dropped it on the S5 12 months later.