I have a guitar campfire songbook which contains folk songs in different keys. Most of them are in D and G, but I also have seen few in Am and C. Now I plan to play the melody with an Asian tuned tremolo harmonica and maybe at a later stage I will add chords with the guitar. Few people stack a 2nd tremolo harmonica above the existing C, which contains all the sharps and it converts then to a full chromatic setup when holding both harmonicas. Would it make sense to play a G folk song on such chromatic setup? It has F#, but not sure if it matters that the harmonica itself is not in key of G.
What you propose is possible, but it might be tricky. Precisely how tricky will depend on the choice of keys of the two harmonicas.
It's easy to fall into the trap of thinking "if I want all the notes to play in any key I might as well just get one in C and one in C♯, just like a chromatic harmonica". Unfortunately, the situation is more complicated.
It is true that a chromatic harmonica can play in any key, but not every key is created equal.
The easiest key on a standard C-tuned chromatic is obviously C, and keeping the button pressed C♯ is just as easy. But what about the other keys? Let's look at the which notes are present in the C and C♯ scales.
C major scale: C D E F G A B C
C♯ major scale: C♯ D♯ E♯ F♯ G♯ A♯ B♯ C♯
To make things easier to argue about, let's assume the convention B♯ = C and E♯ = F, yielding the following.
C♯ major scale: C♯ D♯ F F♯ G♯ A♯ C C♯
Now, compare this to a G major scale.
G major scale: G A B C D E F♯ G
All notes but the F♯ can be found in the C scale, but only the F♯ and C can be found in the C♯ scale. The implication is that each time you switch to the C♯ harp, it's most likely to play a single note and then switch back. This makes it hard not to make the playing choppy.
Even though the keys share so many notes, many keys are easier than G major to play on a C chromatic, and D is even harder! (For somewhat technical reasons, keys with flats are much more convenient to play on a chromatic, but this answer is already long enough so I won't go into specifics..)
So what to do?
If you are not dead set on being able to play in any key, I would consider using two harmonicas in the keys of C and D. Take a look at the D major scale:
D major scale: D E F♯ G A B C♯ D
For songs in C or Am, you can stay on the C harmonica. For songs in D or Bm you can stay on the D harmonica. When playing songs in G or Em you might need to switch back and forth, but contrary to the C-C# setup you can often stay on one harmonica or the other for several notes which makes it much easier to play smoothly.
For playing campfire songs in the keys you mention, I think this is the best approach and you should have an easier time learning to play it well.
If you feel that you need one more key, you could go with harmonicas in C and A, which would make these two keys easy and the keys of G and D intermediate. In any case, it would still be much easier than playing any of these keys except C on the C-C♯ combination I outlined first.
Finally, another consideration is that Asian tremolos are rather limited when it comes to chords: they basically go all in on the home chord. With the C-D setup I suggest, you would have the chords C major, D major, D minor and B minor. There are other harmonicas that would let you play, and while not harder to play they are non standard.
You could, for instance stack two so-called spiral tuned diatonic harmonicas tuned to C and D on top of each other. This would give you:
- Same easy scales as in the C-D setup outlined above (though they are not played with the exact same patterns).
- All the chords C, D, Dm, Em, F, F♯m, G, A, Am and Bm!
- If you learn to bend notes, which is not too hard, this setup is fully chromatic, which means you can play all the sharps and flats.
- The bending that is possible on a diatonic allows you to play some cool bluesy notes if you want.
Spiral tuned harmonicas are a bit harder to find, but they are at least available from Seydel (under the name Zirkular).
Since there's only one 'chromatic' note that needs the button pressed in - F♯ - to play every note in the key of G as opposed to key C, it's not an onerous task. When you need that F♯, just play the F♮ hole, and press in the button. Voila, the harp is in tune for key G. Even in key D, there's only one other note to change - C♯ - which is every C blown with the button in.
Playing chords is a different matter, with really only a C chord available, which will be featured in key G, but not that often, and it may sound odd without the tonic and dominant chords playable - button or not.