Merge two domains and setup ADFS with Office 365

We've taken over IT administration of our sister company this year.  I put off merging the domains but time has come.  Also, we both have Office 365. I'd love to merge these for easier administration.  And lastly, we are implementing Dynamics AX in 2016 and SSO/DirSync will be necessary.  

Any advice as to where to start, caveats, and resources would be appreciated!