Multi-tenant access - Single Sharepoint

Hello,

I've a customer that recently acquire another company. Which one have is own O365 tenant.
Both companies have ADFS and DirSync.
They have a trust relationship between domains and are using FIM to synchronize objects between ADs
Company A wants to access Sharepoint Online of company B without losing any feature.
I've been looking for the following article but what they want is that users are not considered external contributors
http://office.microsoft.com/en-gb/office365-sharepoint-online-enterprise-help/share-sites-or-documents-with-people-outside-your-organization-HA102894713.aspx?CTT=5&origin=HA102849864

How can we achieve this?

Thanks,

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ALN