Hi zebrawebdesigns and buzztone!
Thank you very much for your responses! Sorry about the super delayed reply, I had a bunch of things come up and haven't have time to work on my site, so didn't properly test zebrawebdesigns' suggestion until today.
It works, and the two columns stack neatly on top of one another on mobile!! The only issue I encountered is that when the contact form is viewed as one column on a mobile, the bottom box of the left column and the top box of the right column are stuck together with no padding between them. I solved this by adding a "margin-bottom" padding to the bottom box of the left column in the @media screen CSS. This works perfectly and looks great! zebrawebdesigns - thank you SO much for the suggestion, I really really appreciate your help! Again, v sorry for the late reply!
Cheers!
Ania