I am planning on setting up a Cor-15 with a 1” return line to my tank. Should I be using 1 or 2 return bulkheads and are there any differences that I would notice if I only have 1 return outlet?
You will get the most performance out of the pump with the least restricted outlet. So in one sense the more and larger your return bulkheads the better. However, there is also something to be said for simplicity. More bulkheads and more plumbing equal more places for potential leaks and more complexity.
As a proponent of the KISS principle, I would lean towards a single 3/4in return bulkhead for this pump. This will give some restriction on the pump but this is not a huge pump and I would eventually drop down to 3/4in simply because this is the largest locline size available and I would want a locline return.
There are good reasons for doing things differently but this is what I would do. If you are drilling through the back I would use a kit like this to build your return nozzle.
I would do something similar as MarineandReef Jaron says, but with a little difference.
I would stay 1" all the way including the bulkhead and loc-line on the inside.
Split the return into 2 - 3/4" fan nozzles on the inside of the tank.
Here is a link to 1" loc-line.