Hi DagfinnCS,
I understand that your HP Officejet Pro 8600 Plus printer is not printing the first two pages when you are using the duplex printing setting. I will try my best to help you resolve this issue.
I suggest Resetting the Printer and that you update the HP Officejet Pro 8600 N911g_n Firmware. Also make sure that your Adobe Products are up to date as well.
I hope this helps, please let me know how it goes.
Thank you,