As for using two cutting borderlines at once, it can be done. In the manga, it is sometimes shown that shinki can use two, or very occasionally three, cutting borderlines in quick succession. (As you surmised, barrier lines have to be consciously maintained.)
Like most everything else having to do with shinki powers, being able to send out multiple cutting lines is also connected to how confident the shinki is feeling at the time. On one end of the spectrum, the shinki who seems to be faltering not only has his borderlines cut through easily, but seems to be unable to send out more than one line at a time, perhaps only mustering more in desperate defence. The confident shinki's lines are stronger, cutting through what the shinki wills, and he can easily send out lines in rapid succession.
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As for using two cutting borderlines at once, it can be done.
In the manga, it is sometimes shown that shinki can use two, or very occasionally three, cutting borderlines in quick succession. (As you surmised, barrier lines have to be consciously maintained.)
Like most everything else having to do with shinki powers, being able to send out multiple cutting lines is also connected to how confident the shinki is feeling at the time. On one end of the spectrum, the shinki who seems to be faltering not only has his borderlines cut through easily, but seems to be unable to send out more than one line at a time, perhaps only mustering more in desperate defence. The confident shinki's lines are stronger, cutting through what the shinki wills, and he can easily send out lines in rapid succession.