Hi Doug, My suggestion to you is to get the book "How to grow world class giant pumpkins". This book would answer a lot of your questions.
Which to cull? That is a decision that all of us face every july. This is what I do, I make the decisions primary on 3 things. 1. Vine position, as a rule I like to have a fruit out around 8 to 13 feet from the stump(on the main). It also is best to have it on the outside curve of the vine. 2. Daily growth rates, you will want to cut off the slower growing one. If its hard to tell which one is bigger, start taking daily measurements and keep track on paper which one is growing faster. 3. The number of lobes on the female. I will 99% of the time go with the pumpkin that had the most lobes as a female flower.
I do not cut off any pumpkins until they are the size of a basket ball. When they are that size you should have thought about it enough to make a educated decision.
There are many other variables to be considered also, Like: male pollen genetics,proximity to other pumpkins in the patch etc....the list goes on.
This is a sport of learning, a lot of it is trial and error. Good luck on your decision....Dean o