Co-topping is when two tops are sharing control in a BDSM scene. A typical BDSM scene has a top and a bottom, with the top in control and/or giving sensation and the bottom giving up control and/or receiving sensation.

In a co-top scenario, two tops are sharing the control in a negotiated way.

More About Co-Top
There are many different situations in which two tops may choose to work together. There are coordinated forms of play, such as two people wielding whips at one recipient. Their are collaborative forms of play where one top may be taking more of an assistant role.

Co-topping can be a way to mentor a new top so they can practice their skills in a supervised way. It can be a way for friends or partners who are both tops to play together in a scene, or it can simply be a way to provide a more intense or complex experience to the bottom.