No, it's not poisonous (toxic), however...depending on the volume of ink thats released, it can harmful to either one of the octopuses if one gets close to the ink or if the ink isn't filtered out in time. A build up can cause some issues. The ink can clogg their gills and they can die from it. But this is only if your filteration isn't good, and if you arent taking the ink out in time. Make sure you use a net to get excessive ink out of the tank. And dont let it release multiple amounts of it, or the volume can increase and be harmful. Dont worry that much though on the ink cuz their in a 55 gallon which isn't so small...just worry about getting them seperated.
I recommend..buying an aquarium built "divider." My friend owns one, and what you can do is place it in the middle of the tank and block off half anf half. I also recommend getting a solid colored divider, not see through so they cant see eachother. Once they get bigger though, you will have to seperate both octo's in seperate tanks eventually.