M2 is a term used to refer to Misha Collins and Mark Pellegrino. It can simply be used as a shorthand term for the actors, or specifically in real person slash to indicate the pairing Misha Collins/Mark Pellegrino. The shorthand came into use following the Asylum Europe: No Rest for the Wicked 2010 convention at which the two actors shared a panel to great success.
See also: Marksha
More information: Real Person Slash