Here's the rule, from the Equipment Specifications Manual:
"Holes or indentations for gripping purposes shall not exceed five and shall be limited to one for each finger and one for the thumb, all for the same hand. The player is not required to use all of the finger holes in any specific delivery, but they must be able to demonstrate, with the same hand, that each hole can be used simultaneously for gripping purposes. Any thumb hole that is not used with the thumb inserted during the approach would be classified as a balance hole."
With the Duo Grip you should be able to grip with the index, middle, and ring fingers at the same time well enough to throw a few shots. -- JohnP