You can definitely sync two separate AD forests with a single office 365 tenant. And you can also use AD FS for each one of those forests. This link shows the overall configuration, including Exchange in hybrid mode:
https://technet.microsoft.com/en-us/library/jj873754%28v=exchg.150%29.aspx?f=255&MSPPError=-2147217396
The thing to keep in mind is that each forest needs to have a unique namespace because user authentication is directed to AD FS based on the UPN suffix. So, you can't split contoso.com across the two forests for authentication. You could have contoso.com
for one and eu.contoso.com for the other if required.
You can also see here that multiple forests with a single tenant is supported:
https://azure.microsoft.com/en-us/documentation/articles/active-directory-aadconnect-topologies/
Byron Wright (http://byronwright.blogspot.ca)