Split doesn't work in align with multiple tab stops

Question

I have equations in an align environment with multiple tab stops, and I'd like to use split to render just one equation number for multiple lines of equations. However, if I have more than one tab stop per line, I get the error

Extra alignment tab has been changed to \cr.
<template> }$\hfill \endtemplate 

Here's an example that works:

\documentclass{article}
\usepackage{mathtools}

\begin{document}

\begin{align}
f(x) &= y & g(x) &= z \\
h(x) &= a & h(x) &= b
\end{align}

\begin{align}
\begin{split}
f(x) &= y \\
h(x) &= a 
\end{split}
\end{align}

\end{document}

and produces the following output.

Here's an example that doesn't work. Note the only difference from the above two examples is the fact that split is being used with multiple tab stops.

\documentclass{article}
\usepackage{mathtools}

\begin{document}

\begin{align}
\begin{split}
f(x) &= y & g(x) &= z \\
h(x) &= a & h(x) &= b
\end{split}
\end{align}

\end{document}

Answer 1

I'd like to suggest that you employ an alignat environment if you need all equations numbered individually, and an equation/alignedat nested environment if you need just a single, vertically centered, equation number for all equations.

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{alignat}{2}
f(x) &= y &\hspace{2.5cm} g(x) &= z \\
h(x) &= a &               h(x) &= b
\end{alignat}

\begin{equation}
\begin{alignedat}{2}
f(x) &= y &\hspace{2.5cm} g(x) &= z \\
h(x) &= a &               h(x) &= b
\end{alignedat}
\end{equation}

\end{document}