Let’s say you have Audience A “retro gamers” and Audience B ” Super Nintendo fans.” It is likely that people in B are also in A. There is an overlap between the two audiences.
Since Facebook is a people’s attention auction system, if you target one Ad Set to Audience A and another Ad Set to Audience B, the ads in both Ad Sets will compete with each other.
This set up will lead to poor delivery of your ads, and you won’t be able to accurately test the ads or the audiences since Facebook will determine which ad is going to be shown more often.
You have two ways to solve this:
1) Create exclusive audiences: if you are testing different audiences, make sure they are different.
2) Consolidate audiences: create a new audience with both retro gamers AND Super Nintendo fans. Then target your ads to this consolidated audience.