After seeing LLOYDHOPANDA's version and Riskabe's take, I figured I could make a parametric version of the dual words optical illusion sign, so people could custom make their own!
The result is this script, which works with most combinations of most capital letters. Symbols and punctuation are hit and miss (besides ? and ! which are obviously not going to work), but keep in mind that you can always download the script and open it in openSCAD to tweak the font or modify the design to your wishes.
The resulting shapes from many combinations of letters will have extreme overhangs, so dial in your support structures before you attempt something extremely long. If you have a good phrase feel free to post a make or remix!
EDIT: the script is now updated to gracefully work with spaces and phrases of different lengths. If both phrases have a space, a blank space will be added; if one phrase has a space (or has ended) when another does not, the letter will be intersected with a block, making it visible only on the correct side.