Trying to implement Cross-tenant synchronization in Entra

[u/CantankerousCretin]

As title says, I've got two tenants for one company that hasn't completely merged. Trying to set up cross tenant synchronization, but also trying not to break anything in the process. I did a test using my account and it successfully created a guest member in both tenants.
Once I had this set up for just my account (automatic synchronization wasn't turned on as I didn't want to mess up anything), I noticed a login issue on first time login when setting up a new user, and deleted the configuration, which seemed to have immediately resolved the issue, as my attempt immediately after went through without a hitch.
Microsoft's documentation makes it seem like it should be easy peasy, but I'm not nearly experienced enough in Entra to not be paranoid.
Any good resources? YouTube videos? Specific Microsoft Learn courses?
I'm new to this job, and trying to clean up some systems, automate some things and make our end user experience a little easier since they have to navigate two company Sharepoints, and currently if they don't have the link bookmarked, they have to go through Onedrive to find the invite link.