There are tricks you can use, such as holding both the C major and G major bars down together, which will leave only the G strings sounding. Similarly, holding G and D will give you just the D strings. This is because the note is common to both chords, but none of the others are.

Very handy for doing a 'dum-ching' or 'dum ching ching' where 'dum' would be the common note (G or D as above) and 'ching' would be the chord of the same name (G or D in the examples).

This can be extended to make tunes a little clearer...

I set up my Hopf 12 bar to mimic accordion basses!