@dodtsair: THIS IS SUPER COOL!
Thank you for suggesting this!
I've experimented further and to answer your questions: Will the reeds will still play? Yes. Will they respond differently than in the normal configuration? YES!! (though a little different than normal)
Before going into details, let's just make sure we're on the same page.
@Brendand and @Triona : I think you both misunderstood the question, or at least you don't interpret it as I do.
On the top reed plate the reeds are facing downwards. Adding another plate with downwards-facing reeds is, like you both pointed out, not only practically very difficult to put together, but would not obviously produce any more sound than a single blow reed. (IF one could get the two reeds to operate in the same slot, there might be some draw notes hiding somewhere, but I have no way of testing this and your intuitions might well be right.)
What I think dodtsair talks about is letting the second reed to operate on the outside of the slot. With the harmonicas I've seen this is not possible with traditionally set-up reed plates, simply because how the slots are located. However: if one removes all the reeds from the draw reed plate and re-attach them on the other side and THEN put it on top of the blow reed plate, each slot would look something like this:
Or possibly like this:
One simple way to test this is by simply holding the plate in place so that the bottom slots align. I've tried this and both reeds DO play. It's not 100% air tight, so the draw reed doesn't sound as strong as the blow reed, but this is surely fixable. The opposite reeds directions (ORD) seems to work better if the plates are somewhat shifted; I think this is because it allows for better air flow, see picture.
Both reed directions seem to work to some extent. The ORD should be easier to build and I think it bends better. The bending observations could be due to less leakage with the ORD, but also the physics intuitively should behave more like a normal single reed.
The point is not the HOW though, but that it ACTUALLY SEEMS TO WORK! Closing off the bottom of the chamber I can play both notes, blow and draw, of the top two reed plates.
The really exciting news though is this:
Both the blow reed and the draw reed will bend!
No matter which of the blow or the draw reed is the highest, I can produce interactive bends on the highest note, but also a bend similar to a valved bend on the lower note. With the reeds tuned in unison, both reeds bend. The interactive bends only seem to work with ORD, but I have not investigated reeds pointing in the same direction enough to completely rule them out.
If this would be realised in a whole instrument and be made to work, this would constitute a new class of harmonica along with the half-valved ones and the x-reeds!
I say "if", because there might of course be some issue that shows up on the way from single-hole prototype that sort-of works to a complete instrument.
The fun has only just begun though!
With a top-mounted draw reed lower than the blow reed TOGETHER with an ordinary draw reed down where it's normally located, any drawing by the player seems to mainly activate the lower draw reed. Blowing, on the other hand, activates the blow reed which may interact with the top draw reed and thus bend downwards. The result is a new kind of x-reed harp!
There are more ideas here, but the post is already long enough.
Once again: Great question dodtsair!