This is a workaround because we don't have an automated way to consolidate cards or write text messages on both sides.
(1) Click on a card in your box to show a large view, right-click the text image and select "Copy image" — which will put it in your clipboard.
(2) Click an existing card that only has a message, select "Add Photo" button and paste (command-V on your keyboard). Don't click on any anything (My Computer, Facebook, Instagram) — just paste and it will add the photo from your clipboard.
Note: The image already has attribution on it, and your name will be in the byline on the other side, but if you want, you can change that byline selecting the pencil in the upper-right corner of the card and change your name to someone else's name.
(3) Delete the original card you copied by clicking on the pencil in the upper-right corner of the card and selecting "Delete Card."