Landscape Tiled with difrrent reseltions?


I am looking to make a landscape with a background landscape that is in total 24KM big.
Where the center 8Km by 8Km. the rest is a background. i want the center 8km by 8km to be a reseltuon of 4033 and the outsice to be 2017. to increase performace but when i try inporting that i get problems i cant towrk on both terrains toghater.
and the blend from world machine is not the best when you export 2 times so i wanted to edit that in engine but i cant.
i can only select and work on hte background or on the middel. does any one know how i can better do this. or can i just export it all,
in a 4033 i realy want to save performance.