How to make an align environment with multiple equations per line and one equation reference? [duplicate]

I would like to use an align environment with a single equation number but with multiple equations per line. For example, the following two pieces of code work:

begin{align}
a&= 1 & b&= 2 + 3\\
c&= 4 + 5 & d&= 6\
\end{align}

begin{align}\begin{split}
a&= 1\\
c&= 4 + 5
\end{split}\end{align}

The second example succeeds in having one equation reference in the middle, and the first succeeds in having multiple equations per line. How might I have multiple equations per line with one equation reference? Thanks!

marked as duplicate by yo', user31729, Svend Tveskæg, T. Verron, user13907 Nov 12 '14 at 17:36

\begin{aligned} a &= 1 & b &= 2 + 3 \\ c &= 4 + 5 & d &= 6 \end{aligned}