Call Marriott customer service, get a rep to merge the accounts for you on spot. I would also make screen shots of all impt info like your reservation confirmations and elite/LT nights trackers.
Make sure you get a knowledgeable rep. My spouse called in to combine and specifically told the rep to use the legacy Marriott account as the new bonvoy number going forward, and yet the stupid agent used the legacy SPG number as the new account. Now we have to get all the points sorted with Chase, as they have issues crediting cc spend for points into the newly combined bonvoy account which the existing marriott member # doesn't exist..