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It's years since I've run the wizard. However, if you take the defaults it will indeed scan all of your drives. The additional external drive will just deliver more results, and will not affect Recuva's operation.
You seem to be asking why Recuva shows so many file when you've wiped the disk and MFT. At a guess (assuming you ran WFS/WMFT on all drives) you're seeing filenames from the MFT. Wipe MFT will overwrite entries in the MFT, not remove them, and Recuva in normal scan mode looks at the entries in the MFT. Or several MFTs in your case. The MFT never reduces the number of entries it holds, so you will always see what appears to be a fair number - thousands - of files.
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What I mean by 'More results' is that Recuva's list will include files from all available drives if you use the wizard, whether they're flagged as recoverable or not (it won't show zero-byte files). So the difference between running Recuva with the ext drive attached and not attached is that more files are listed with it attached - as you'd expect. The list will show which drive the files come from.
I thought you had already run WFS/WMFT on the ext drive. Of course it's entirely up to you whether you do or not. You will still get the same number of results from a scan but the MFT names should be gibberish. Or should I say carefully selected by experts at Piriform.
You'll also find that you can recover unrecoverable files (it's not an accurate name for them but it will do). However the data, probably binary zeroes after a WFS, is of no use to anyone.