They have quite a few that they tend to be on opposite sides of! They're both into BDSM; Mobo likes being tied down while Robo enjoys doing the tying. Robo really likes being used as an object by his brother, too. Basically, they can only really be completely vulnerable with each other, as they trust each other completely, so they push their own boundaries and experiment a lot. Robo in particular is often trapped inside his own head, so being able to empty it out for a while and just focus on receiving sensation is really good for him.