In Gmail, let's say I have 7 contacts in Group A and 9 contacts in Group B.
2 contacts are in both Group A and Group B.
Is there a way I can filter to get results of only the contacts that are in both groups?
No
What you're looking for is the label: filter for the search bar.
It works on the mail/inbox section of Gmail but apparently hasn't been implemented in contacts yet.
If it worked you'd narrow the results by typing the following into the search bar:
label:Group-A label:Group-B
I have a workaround for this:
With these basic operations you could achieve the desired functionality as follows:
And voila! your result using only 3 temporary groups and 8 group manipulations.
If you´re just trying to make sure that there is no overlap between two different groups you can go into either group and select it all. Then click the group button as if you were about to add them to a different group and it will have a dashed line next to any other groups that any of those contacts are in