Step 1: Select the "client" vSAN cluster (the one that wants to mount the other's datastore) and click on Configure. In this setup, both vSAN clusters are managed by the same vCenter server as you can see and happen to be in the same Datacenter.
Step 2: Under vSAN, select Datastore Sharing like shown below. Notice that two clusters exist in this case and both are controlled by the same vCenter Server.
Step 3: Click on MOUNT REMOTE DATASTORE
Step 4: Select the remote datastore and click on Next.
Step 5: Verify that everything looks good and click on Finish. Notice the two captures. One shows you why it may fail, the other one after fixing whatever may be incompatible with HCI Mesh.
Verify that you can access both datastores from one cluster. Feel free to storage vMotion vms between the two datastores. Use commands like df -h or the gui to look at more information. According to Cormac Hogan, one cluster can mount up to 5 remote vSAN datastores. Be aware that HCI Mesh requires Enterprise or Enterprise + licenses.
Useful YouTube video: https://www.youtube.com/watch?v=Dcdwy1wh-DM&feature=emb_title