Conditional formatting based on element wrapping!

Hi,
In a row container with two elements, the second element has max width 50% predefined. I would now like to apply a responsive condition to this element that while reducing screen width, once the min width of both elements is reached and the elements wrap, at that moment the max width of this element becomes 100%.
The only option I’m getting is to use condition of page width < 769.
Please suggest how I can achieve this.
Thanks & regards,
Sutwala