When Marriott’s acquisition of Starwood first closed, they immediately allowed you to link accounts. That gave you the ability to status match and it gave you the ability to move points between accounts at a ratio of 1 Starpoint to 3 Marriott points. Linking accounts does not and will not ever automatically combine them.
Marriott tells me they are “now rolling out [the ability to combine accounts] in batches to our members through tomorrow. As a result, when members log in and see the prompt they will have the opportunity to combine their accounts.”