I believe it would work as follows:
You will have one 8949 showing the results of your Import.
You will have another 8949 showing the summaries from the second broker [update: see next comment].
If they are short term trades, there is no need to send any mailing and you should not see a Code "M" on page 8949.
But Turbotax may be thwarting you, if you do see a code "M" it is telling you to mail paper trail to document that subtotal .
All your subtotals have to end up as a grand total on Schedule D in any case .
