I experimented with your setup, i.e. blank p/w's for both my main and test accounts, with these results:
When I logged out of either account I was presented with a list of users/password dialog box, and selecting a user automatically logged me into whichever account I selected...with no further ado.
Your post sounds like that's not what you're experiencing, though. Sorry if I haven't been clear, but that's exactly what happens to me.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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