What would the plot of an two sided digital signal look like?
A function of $x$ is right-sided if and only if there is some
$x_0$ such that the value of the function is zero for all $x &lt;
x_0$, and left-sided if and only if there is
some $x_0$ such that the value of the function is zero for all $x &gt;
x_0$.

Likewise for signals with time represented by $x$. A double-sided signal has neither restriction. For example a Gaussian function and $\sin(x)$ (both pictured below) are double-sided.

$f(x) = \frac{e^{-\frac{x^2}{2}}}{\sqrt{2 \pi }}$

$f(x) = \sin(x)$