Yes, they are in different server site. They are 2 different and full ArcGIS enterprise 10.7 each with its one primary datastore.
I wonder if I can connect the two servers to the same datastore (to use as secondary)
I assume you're referring to the ArcGIS Data Store, and no, it doesn't support being registered to two distinct sites. You'll need to set up two separate servers, install the ArcGIS Data Store on each, and configure them as separate standby Data Stores for each primary.