Best course of action, just to be safe, would be to do strand tests before any of it touches your head. That's particularly true if you're picky about the color.
But there's no reason why you can't henna first, then switch to henndigo. Just realize that the permanence, particularly of indigo, varies tremendously. Many people find it fades, a few people find it won't ever fade, and many find that if they apply peroxide to hair that apparently hadn't had all the indigo trimmed off, it goes greenish. It's sort of fiddly, but things like overlapping root touchups tend to be very forgiving.
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