Hi, in order to sync cross tenant, there is a new B2B sync feature that should be rolled out that allows for this. However, B2B requires AzureAD accounts, so using gmail accounts or MSA accounts will not work.

For more information regarding the feature check out this article: https://docs.microsoft.com/en-us/onedrive/b2b-sync

Sorry don't have the answer I'm sure you were wanting.