About Interface between nRF52840 and nRF9160

[Andy kang]

Hi, I'm designing water meter modem using the nRF52840 + nRF9160.
I will apply nRF52840 as a HOST CPU and the nRF160 contol by nRF52840. (please refer to picture.)
Which Interface can I connect which Interface between nRF52840 and nRF9160?