I've never seen one on the market on a standard 6-hole whistle design, with a double-hole added.
One consideration I'd wonder about is the ability of the player to have good control accessing
a double hole on a metal whistle or high key whistle tube, whereas, on a plastic bodied recorder, the double hole positions
are flattened down for both holes so that the finger tip can easily slide across one or both holes.
One a 6-hole whistle in the key of high D or going down to perhaps alto A, the curvature of the tube surface
would put the double holes at different horizontal positions, and the ease of controlling access to both
positions, quickly, might be an issue. I've got a Yamaha recorder right here, and I'm relaying this issue
as I see it. On a wider body whistle, such as "perhaps" alto G or lower keys, the wider body with therefore
more of a flattened surface, might be more agreeable as a experience making agile motions on double-hole air channels.
You might want to contact the Thornton Whistle folks, who on their whistles do flatten the surface
of the whistle at the positions of holes (all 6 of them), which might remove that issue, at least. The
thicker wall wood body of their whistles allows for that alteration. Sliding a finger tip across
that area would be simple, and Thornton Whistles would make a killing by having a monopoly on the market
of the only chromatic or near-chromatic whistle, now that Bracker is for some reason as yet still unknown,
not yet back in production, with their 9-hole whistles which ARE chromatic. The Shearwater 8-hole "recorder C" whistle
is almost fully chromatic, but only available in the key of C, a fine key with a lot of public acceptance,
I like C major more than nation states and religions, but I wish they offered that chromatic feature on altos and lows as well.
Take a look at wise innovation in whistle design:
Thornton Whistles:
https://tommmymartin.wixsite.com/thorntonwhistles