HELP please merge back up data

How do I merge my iphone 6 back up info with my iphone 6s. When I got my 6s it only let me restore from Dec 14-Jul 15 and didn't let me add Aug-Dec 15 to my new phone. To complicate matters further I've been only using my 6s and don't want to lose anything from Dec 15-until now. So my question is I've now backed both phones..... Now how do I merge them???