I have no idea, but I would worry that copper is used in some pigments for blue-green colors, so what's normally thought of as a "harmless pigment" might be a problem no matter what the base of the paint is. But keep in mind the "I have no idea" part, this is more intended to prompt people who actually know stuff...
Animal Mother... how thick was the acrylic you used? Just curious since I was planning to use it for a cover myself. However, I don't think bowing would be a problem for the application that simple has in mind with the sheet oriented vertically.
simple... you might consider just buying colored acrylic to save yourself the trouble and worries of painting. USplastic.com is a place I've purchased from in the past. They have acrylic sheets in a variety of colors.
The pieces I used were about 1/2 inch thick. Vertical orientation might not cause a problem but he would still have to color them to keep the octos from stressing each other out, so I still suggest acrylic.
Consider, octopuses have been kept in divided tanks before... a specially designed, very well planned out, EXTREMELY expensive tank comes to mind... and the octopuses still outsmarted the design and got into one side together. One killed the other. Search for threads by member Goodwin9. His tank was VERY elaborate, still didn't work out.
i was planning on using two sheets of acrylic, each 8 inches apart. Then i was going to drill hundreds of tiny holes in each to keep the flow going. Except i would make the holes be offset so the neither octopus can ever directly see the other octo. I will also make separate lids for each half of the tank. I was planning on making the dividers removable using the instructions from this link: http://www.wetwebmedia.com/ca/volume_3/cav3i1/Baffles/baffles.htm