Yea, in his mixing video he adds Part A (doesn't show if stirred/mixed), then Part B (doesn't show if stirred/mixed, technically wrong order), then Epsom. As you pointed out by saying you put Part A and Epsom in the same bucket, there's already "Epsom" in Jacks (Part A), so it's fine. It just means you can't then alter those ratios later (except if you wanna add more Epsom, I guess).
I'd do it in the right order though it's easy to not think of it, if you didn't know, because you'll want to add it in the order you're weighing it (as a newb), and if you're mixing any type of concentrated mix to then dilute, you'll probably run into issues (precipitation).
This is jacks actual instructions on order of mixing. They used to have an info page that was great (shown in the video you linked, now I can't find it).
This video has even more great info in it: "Greengenes Live: Nutrients" (don't wanna post more vids here).
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