According to you, it appears you are trying to make a double-sided copy, as indicated by this statement: "I'm trying to make a copy of two separate papers into a 2 sided copy (front one page & back side the second page)." The steps I provided above are intended to enable this setting. I am not referring to printing from the computer.
For more information on this, please look at the product manual: HP Officejet Pro 8600 e-All-in-One Series Printer Series.
Now, if you're referring to double-sided printing, then please follow this HP document on Printing on Both Sides of the Paper (Duplexing).
Please let me know if you have any further questions.