Desired output:
Here is an MWE which achieves exactly that:
\documentclass[11pt]{article}
\usepackage{tikz}
\usetikzlibrary{calc,intersections,through,backgrounds,matrix,patterns}
\usepackage{pgfplots}
\usepgfplotslibrary{fillbetween}
\pgfdeclarelayer{bg}
\pgfsetlayers{bg,main}
\makeatletter
\pgfdeclarepatternformonly[\LineSpace]{diagright}{\pgfqpoint{-1pt}{-1pt}}{\pgfqpoint{\LineSpace}{\LineSpace}}{\pgfqpoint{\LineSpace}{\LineSpace}}%
{
\pgfsetcolor{\tikz@pattern@color}
\pgfsetlinewidth{0.4pt}
\pgfpathmoveto{\pgfqpoint{0pt}{0pt}}
\pgfpathlineto{\pgfqpoint{\LineSpace + 0.1pt}{\LineSpace + 0.1pt}}
\pgfusepath{stroke}
}
\pgfdeclarepatternformonly[\LineSpace]{diagleft}{\pgfqpoint{-1pt}{-1pt}}{\pgfqpoint{\LineSpace}{\LineSpace}}{\pgfqpoint{\LineSpace}{\LineSpace}}%
{
\pgfsetcolor{\tikz@pattern@color}
\pgfsetlinewidth{1pt}
\pgfpathmoveto{\pgfqpoint{0pt}{\LineSpace}}
\pgfpathlineto{\pgfqpoint{\LineSpace + 0.1pt}{-0.1pt}}
\pgfusepath{stroke}
}
\makeatother
\newdimen\LineSpace
\tikzset{
line space/.code={\LineSpace=#1},
line space=8pt
}
\begin{document}
\begin{tikzpicture}
\draw[name path = axis] (0,5) node[below left]{1}node[above]{$S_2$} --(0,0) node[below left]{0}--(5,0)node[right]{$S_1$} node[below]{$1$}--(5,5)--(0,5);
\draw [name path = B1, thick] (0,5)--(1,5)..controls (3,4) and (3.5,2)..(4,0);
\draw [name path = B2, thick] (0,5)--(0,4)..controls (1,3.5) and (2,1.5)..(3,0)--(4,0);
\begin{pgfonlayer}{bg}
\fill [pattern= diagright,
intersection segments={of=B1 and B2,sequence={L2--R2}}];
\end{pgfonlayer}
\end{tikzpicture}
\end{document}
The control of pattern 'density' (note the use of pattern = diagright rather than north east lines) was achieved using @Red's answer here. Specifically, it is controlled via the dimension of \Linespace.
I am currently in the process of making a template for LaTeX newbies to follow (this particular template demonstrates how to shade between lines). However, they are often intimidated by modifications like the one employed here.
Noting that Red's answer was in 2013, is there a new, simpler way to achieve this goal of controlling pattern density? Maybe something like pattern density = X?
