Having multiple files open simultaneously in PSE 8.

I cannot get PSE 8 to have 2 files open side by side so that I can compare them for manipulation.  With the first file open in the work space, double clicking a second file from elsewhere e.g. the project bin PSE 8 replaces the first file with the second file and closes the first file.

Can anyone please suggest a solution which I beleive has only now started occurring.

I am running PSE 8 on an iMac with OS X El Capitan.

Sincerely, ranger123.