Sorry - I did not know what version you had. Now you know what to look forward to when you use 15.1
You are correct that you cannot have two plane diagrams in one design folder. But you can have another design folder and one plane within it..
For 14.1 you will lose the reference circuits for any harness items that you copy and paste. Therefore I suggestion:
1) Select Design folder (just about the diagram) of the biggest plane that you wish to merge
2) RMB - Copy - make a new Design folder which will contain this new Plane. The links to the reference circuits is kept.
3) Now select the data in the other planes and Copy the items.
4) Paste using Ctrl+Shift+V so that you keep the same names of the objects.
5) Now you will have to Associate the additional Logic reference circuits. But if the Devices in the Logic diagrams are the same as the Slots then Actions - Place - Auto-Place Devices will quickly take care of placement for you.
Because you are merging planes, I would suggest DRC checks for duplicates before doing placement. Other checks too as you feel needed to maek sure the data is aligned.
Regards,
Pat.