Linking 2 wp site on the same host..

Hy, if somebody know’s how to link 2 site please post

I want to link 2 site in wp .
The site are on the same domain

Ex: https://www.diviziatm.ro -primary
https://www.casa.diviziatm.ro – second.

when I add a post on casa.diviziatm.ro i want to be desplay on the primary…

Thamks