As for casting multiple shinki spells, yes, that is possible. Since the name of the target is part of the spell, a shinki cannot cast spells that affect multiple targets simultaneously -- in other words, a shinki can't say "Umbra, A and B!" However, it's possible for the shinki to cast spells in quick succession, so she could say, "Umbra, A! Umbra, B!"
This does require more experience, focus and comfort with the spell than simply casting one spell on one target, and the duration of the effect will also increase with experience.
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As for casting multiple shinki spells, yes, that is possible. Since the name of the target is part of the spell, a shinki cannot cast spells that affect multiple targets simultaneously -- in other words, a shinki can't say "Umbra, A and B!" However, it's possible for the shinki to cast spells in quick succession, so she could say, "Umbra, A! Umbra, B!"
This does require more experience, focus and comfort with the spell than simply casting one spell on one target, and the duration of the effect will also increase with experience.