Most bowlers use a finger tip grip which has three holes, with the thumb all the way in one hole, and just the first joint of the middle and ring fingers in the other two holes. A finger tip grip is easier to get out of (to release the ball) and puts more rotation on the bowl.
Having said that, if their is a way to drill (or not) and throw a ball someone has tried it. There are bowlers using no holes, some using two for fingers only; and their are even bowlers that go finger tip for the fingers and use just the first joint of the thumb - kind of a more stable to hold version of no thumb hole at all..
Finally, some bowlers that started throwing two handed when little kids have continued to throw with two hands very successfully as adults.
You really need to work with a ball driller who can see you throw the ball, to get what's best for you and your style.