Collecting Single OR Double Key Presses

I sent a long-winded e-mail a few days ago, but just realized I could sum it up in one question:

Is there a way to collect single OR double (i.e., two simultaneous) key presses in a given trial?

Thanks in advance.

To make sure I'm clear on what you mean by "or"... Do you want to give them the [I]option[/I] to respond twice, but allow them to respond only once if they so choose?

Exactly.

I want them to have the option of replying with either a single key press, or with two simultaneous key presses.