Tiled landscape import problems

Hello. I am trying to import 22x22 km tiled landscape into my project. Total resolution of tiled landscape is 16 144x16 144, one tile has resolution of 1009x1009. It gives 256 tiles. Problem is, that when I am importing all the tiles, editor crashes at 10-20% of tiles. My idea was to import it all in 16 parts. It worked. Terrain looks good, and everything works fine, but now I have 16 different landscapes which is hard to paint or sculpt (or is there any way to combine them into one since they are all the same size?). My question is how to import it all at once and not crash the editor? I will be very, very grateful for any help.