I don't see a tab for setting "Default" breakpoints in RED... there is one in Site Designer.
I really can't explain it any better than this video: https://youtu.be/rjp0cInhoDA
This sample video was done with a competitor's email builder.
I have hand-coded this in the past but I am trying to bring all my clients into RED. One client, in particular, has all their emails done this way with no exceptions.
So my workaround is to add a third row and hide it with the "Display" toggle set to "None" till I get to the mobile media queries and then clear the "Display" toggle and then hide the offending 2nd-row columns (you can't hide a full row with the "Display" toggle with media queries per the developer and my testing).
Not exactly how I wanted to do this but it works.
Video proof: https://youtu.be/RzAI336tDf0