It is common knowledge that you should never mix the two types.
Call me paranoid, but here it is from a lawyerly perspective: You know it's a no-no to do this based on the unpredictable handling characteristics...If you got into an accident with that car tonight, the insurance company would most likely deny your claim based on the fact that you contributed to the risk of an accident by installing the mixed tires...and the lawyer(s) of the person you had an accident with, or the innocent bystanders, etc, whether it was your fault or not, would have a field day bringing the mix and matched tires up in court. They'd make that the entire issue of the case, not the fact that somebody ran into you, but the fact that you put 2 radials and 2 bias plies on the car.
Remember the O.J. trial??? If the tires dont fit, you can't acquit.
I'd stick to all radials or all bias ply and avoid the risk altogether.