When engaging a bad guy, showing up with friends is always a good idea. To maximize your advantage, you should be striving to make your job easier and his job tougher. A basic way to do this it to strive for as close to a 90 degree angle of convergence on your opponent as possible.
While having a buddy by your shoulder isn’t really “BAD” per se and many shooting instructors train to work in close proximity to a cohort (it’s also easier and safer as far as range safety/control goes), if you are too close to your partner you are not making it difficult enough for the opponent to engage the two of you simultaneously. All he has to do is adjust his sight picture slightly to left or right to engage both targets.
If you think about it (when the ratio is 2:1), there is an inverse relationship of angle to advantage going on here. The wider the angle is between the two “good guys”, the more difficult it is for the “bad guy”.
The “bad guy” has to track and target threats that are farther and farther apart, making it more complicated to OBSERVE and ORIENT on his opponents.
Ideally, the optimal configuration is to attain a 90 degree angle on the bad guy. Of course all depends on time, distance, cover, terrain and other variables. Always remember that “best” can sometimes be the enemy of “good enough”.
Once you get past 90 degrees, the danger of hitting each other increases.
Something to think about: The inverse of this lesson. If you are outnumbered, you should be trying to maneuver yourself so that your opponents are at a narrower angle of incident.
Musashi had this to say on the subject:
Waiting is bad. Always quickly re-assume your attitudes to both sides, cut the enemies down as they advance, crushing them in the direction from which they attack. Whatever you do, you must drive the enemy together, as if tying a line of fishes, and when they are seen to be piled up, cut them down strongly without giving them room to move.