These two had something special, something neither of them had before. Genuine, close, understanding friendship. The kind of friendship where you genuinely feel like a whole with them and like a part is missing without them. Any friendship bracelets, necklaces, always broke. No matter what they broke. So, the two got a tattoo. In a specific place of a specific thing.
Tengen has a flame tiger on his right wrist, and Rengoku has Tengens name, but its sheet music for a clarinet and if you were to play it, it would sound like your saying his name. Each one holds a very specific meaning to the other. That tattoo is the reminder of their other half. And while people have seen it, they never put the two together. Somehow.