I noticed following from Under the Hood - vSphere 6.7 Update 1 - VMware vSphere Blog
Now, you have the flexibility to combine, merge, or separate vSphere SSO Domains ...
So it is already possible to MERGE SSO domains? For example, I have 2 independent deployments of VCSA 6.7U1 with their own "vsphere.local" SSO domain. Is it possible to merge them into 1 "vsphere.local" SSO domain so that ELM can work?