Any way to cut a large terrain into multiple small terrains?

[Wuyan]

Well, I'm editing an 8km * 8km world terrain. I have a problem. If I divide them into many small terrains at the beginning using world designer, the editing process will be very slow. I want to edit an 8km terrain directly first.  When editing finished, cut it into many small terrains. I didn't find a good way. Scanner is a selection,but will lose some details due to resolution limitation. Is there any other better way?

In addition, the scanner cannot save multiple terrains data to one stamper, right? Although I drag in several terrain, it only saves one terrain data at a time.

 

Thank you!

[Bryan]

Thats correct as Unity doesnt allow multi-terrain editing. 
Sadly this is the only way to do it.