How can we migrate Yammer across two O365 tenants ?

I don't believe this is initially available. This means you have one Yammer network that is associated with only one Office 365 tenant. See Microsoft's documentation on 1:1 Tenant. 

See more info here from Microsoft: https://support.office.com/en-gb/article/Yammer-and-Office-365-Groups-d8c239dc-a48b-47ab-b85e-6b4b8191a869