Metta and Loving-Kindness as a Team Can Metta be practiced in a partnership or as a team? 

I can think of


two people practicing Metta together, or 
a group which meets for meditation, or 
across borders, across continents people meditate together at a certain time.


Are there any indications of a Sutta?

Are there practices of a tradition?
Meditation influences the surrounding by vibration created. (You can really feel the vibration when you experience Piti) So when you pratice as a group these vibrations influence others, especially when you are having a bad patch in meditation then the influence of others will help in to develop calmness and concentration and perhaps dispel agitation and other hindrances that might have surfaced during your worldly interactions.

This is true for all types of meditation but most effective when doing Metta as vibrations tend to be stronger.