In bubble you cannot change group or in traditional way DIV , layout , if they are set to colum , its childern will be stack as colum and if they are set to row, its childern will be stack as row:
what you can do , make the parent group row, and in his childern give a minimum with in Layout tab of small editoty like 320, this way if screen get smaller to 320 it will stack all its childern verticaly.
You don’t need to use percentage in this case; set a minimum width in px and a maximum.
Since the phone’s width is smaller, that’s when you use the breakpoint condition, and then instead of the two groups on the same line, through the condition you set them to have 100% width; that way they fill the line completely, making the other group go below and fill its entire line.