GAL Federation between different O365 tenants
As organisations grow, or have to separate different business areas in different O365 tenants it would be useful to be able to share or federate global address lists between different tenants. The only current choices are:
Create contact objects in tenants which are then disconnected from the original user - this means if the user details change the contact in the other tenant becomes out of sync.
Where domain federation in in place and directory synchronisation services running for each tenant, create on premise AD users as mail enabled users and ensure each directory sync for each tenant looks at the other domains. This will ensure that user objects are sync'd to the other tenant as mail contacts.
The second method works quite well from a point of ensuring that user details are always accurate with AD objects but only works in an environment where domain federation and dirsync are used. This is not an option in a completely cloud based environment.
Allowing GAL federation/sync between tenants would reduce on premise administrative burden and also provide a solution for cloud only adoption.
This. Is. Really. Important.
It should be possible to federate cross-tenants as seamlessly as possible. I don't know if Azure AD B2B will help or go what distance in realizing this. But, sharing contacts is unwieldy.
I, as many others are, am also seeking to share GAL between two O365 Tenants. Based on my research Microsoft does not provide a way to do this as the OP has pointed out. The workaround (#2 above) has potential as long as your two orgs have on prem AD and can be connected in some way (VPN/MPLS/etc).