You cannot do this with the actual version.
However, we like your suggestion, so we slightly change the syntax with the following beta:
You can now use -- or || to denotate concurrent states.
Unfortunalty, this slightly change the rendering for old diagrams with concurrent state because -- denotates now a horizontal separator (sounds more logical)
However, thoses diagrams will still working, and the correction (change -- to ||) is minor, so we decide to go for this new syntax.
Thanks again for your suggestion, and feedback welcome!