Gonna do my best to explain how you can "stack" two Utsu:Ichi...
You cast Utsusemi: Ichi. You now have 3 shadows protecting you. Make sure you do this right after your previous fight ended, so the recast timer is up soon. When the puller gets back with a mob, Provoke it. Then watch the chat log for when your shadows are destroyed. When you have only 1 shadow left, recast Utsusemi. It's easy to know when you lose a shadow, because the "shadows absorb damage" message is one of only two chat messages that begins with a number (the other chat message is after any sort of HP/MP/TP drain ability). The cast time is slightly longer than the mob's attack speed, so you'll get attacked 1 more time while recasting, killing your last shadow. However, you'll finish casting right before the mob can attack you again, giving you 3 more right away without you taking any damage. The only reason you can't do this as easily mid-fight is because of the recast time - you need to wait 30 seconds to recast. It's easy to do this at the start of a fight because you can cast Utsusemi and let the timer run down before you start losing shadows.
Two side notes: first, mobs with double attack will mess you up when counting shadows. Don't be surprised if you lose your last shadow and get hit before you can put up more. Second, note that if a mob misses you, you don't lose a shadow. So it's possible to keep the one shadow you have left when you recast Utsusemi. However, this isn't a problem - the new Utsusemi will replace the old one, and you'll still have 3 shadows right away.
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