In my view:
1. It will only resist torsion by warping of the flanges due to a partially broken load path (see next point).
2. It is unable to resist St Venant Torsion i.e. J=4A0/(ds/t) type for a box section. For a single box there is a broken load path for the circulatory shear forces.
3. It will be able to resist a minor axis moment, but not be able to resist a major axis moment.
If you want to weld it that way I'd be very careful with your support conditions in your model (and preferably weld it all around instead).
gl