Two

Left and right make a pair.
Where there's pitter, there'd be patter.
You can't have an exchange on the solo.
You hurry on the double.
A dialogue requires two parties to communicate ideas.
Have you heard of a single-headed snake? No? I didn't think so either.
Engaged with another in a tussle confines a duel.

Two?
Two's too much trouble, if you ask me.

=.=