Team_enemy normally handles shooting buddies better than team_player, so in that sense you are lucky. You might just try set_enemy("targetname"). In theory, it should be enough. However, you should also realise that once either of the two STs kills the other, he will likely turn his attention to player. So, it might be worthwhile to set them neutral (unless you want them to start shooting player once their internal quarrel is over). Or if it better suits your purpose, you can, of course, set the team of either of the faction to team_player, which will easily solve the issue.
But having two large team_enemy groups fight with each other is difficult, because JA does not allow other groups that _player, _neutral, and _enemy. No subgroups.
Frozen in the past by ICARUS