Yes you can absolutely do this. Use the AF PNR on DL.com and you'll be able to select seats and update to your DL FF # if you want and then you'll always see it in "My Flights" on DL.com

Did exactly this earlier this year, although for D1 not Y, but the mechanism is the same
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In terms of booking I would book simultaneously in different incognito windows to minimize chance DL raises price on DL award after the FB one is purchased (though submit FB first).