If there is a strong possibility that squirrels or raccoons will enter a chimney...
have a chimney cap installed, preferably one made of stainless steel.
Below is a picture taken on the job.
Notice the multi flue chimney.
When two flues are in close proximity, this makes installing two separate caps impossible.
Obtain a Forever Stainless Steel Chim Cap to cover both flues at the same time.
Chim Cap Link:
http://www.chimcapcorp.com/home.html All products are made in the USA.