There is a thread further down.
Both accounts remain separate.

Here my case:
I was SPG Gold and MR Plat. I have linked both accounts - now I am MR Plat and SPG Plat.
I still have my MR account and my SPG account. Both accounts are still separate !!!
If I sleep at Marriott properties, the points and nights will go to MR.
If I sleep at Starwood properties, the points and nights will go the SPG account.

And there is a possibility to transfer points (ratio 3:1).

In 2018 both accounts will probably be merged to one MR account.