The Minstrels on my server are able to use Ensemble(duet) songs by themselves. I didn't make this change.
This is my skill_unit_db
307,0x9f, , 4, 0, -1,enemy, 0x220 //BD_RICHMANKIM308,0xa0, , 4, 0, -1,enemy, 0x200 //BD_ETERNALCHAOS309,0xa1, , 4, 0, -1,party, 0x200...