Form with left / right columns flowing as one column in mobile

Hello everyone,
Currently, this is what I have on my form:

form id=“form1”
div class=“form-row”
div class=“col” (column left)
form input fields
div class=“col” (column right)
form input fields

Which displays the following results on any type of device, desktop, mobile, etc:
Col-left form input fields | Col-right form input fields

QUESTION
When using a mobile device, how can I make the form input fields on the Col-right move below the Col-left form input fields? Like this:
Col-left form input fields
Col-right form input fields

Thanks for the feedback.