Is it possible to right-align the second line inside a "split" environment inside an "align" environment?

Question

When breaking a long equation over exactly two lines, the multiline allows doing so without adding spurious equation numbering and without manually placing \notag tags. It also right-aligns the second line.

When needing such a line break inside an align environment, a nested split environment can be used. But when using this to introduce a line-break into the right-hand side of the equation, it will be left-aligned.

Manually aligning the equation with \phantom{},\hspace{} and \qquad constructs is doable, but results in less readable and less maintainable source code, so I want to avoid it at least until final editing.

Is there any way to get it right-aligned instead? Is split even the correct environment for this usecase?

MWE

\documentclass[12pt]{article}
\usepackage{amsmath}
\textwidth = 10cm
\begin{document}
\begin{align}
  The =& First Equation Is Short Enough For One Line
  \\
  \begin{split}
    The =& Second Equation Is So Long That Splitting It
    \\   & Over Two Lines Is Desirable
  \end{split}
\end{align}
\begin{multline}
  But = How Do I get The Second Line To Be
  \\    Right-Aligned Like For A Multiline?
\end{multline}
\end{document}

Example from real-world document

  \documentclass[12pt]{article}
  \usepackage{amsmath}
  \usepackage{fouriernc}
  \begin{document}

  \begin{subequations}
  \begin{align}
  \Delta_1B(t)
  &=
  \sum_{\alpha}
  F_\alpha e^{-i\omega_\alpha t}
  \sum_{ij}
  -\frac{
    f_{ij} B_{ij} A^\alpha_{ji}
  }{
    \hbar(\omega_{ij}+\overline\omega_\alpha)
  }
  \\
  \begin{split}
  \Delta_2 B(t)
  &=
  \sum_{\alpha\beta} 
  F_\alpha F_\beta e^{-i(\omega_\alpha+\omega_\beta)t}
  \sum_{ijk}
  \Bigg(
  \frac{
    f_{ik} B_{ij} A^\alpha_{jk} A^{\beta}_{ki}
  }{
    \hbar^2
    (\omega_{ki}-\overline\omega_\beta)
    (\omega_{ji}-\overline\omega_\alpha-\overline\omega_\beta)
  }
  \\&
  +
  \frac{
    f_{jk} B_{ij} A^\beta_{jk} A^{\alpha}_{ki}
  }{
    \hbar^2
    (\omega_{jk}-\overline\omega_{\beta})
    (\omega_{ji}-\overline\omega_\alpha-\overline\omega_\beta)
  }
  \Bigg)
  \end{split}
  \end{align}
  \end{subequations}

  \end{document}

Answer 1

I can suggest a pretty hacky way with multlined from the mathtools package:

\documentclass[12pt]{article}
\usepackage{amsmath,mathtools}
\usepackage{fouriernc}
\begin{document}

\begin{subequations}
\begin{align}
\Delta_1B(t)
&=
\sum_{\alpha}
F_\alpha e^{-i\omega_\alpha t}
\sum_{ij}
-\frac{
  f_{ij} B_{ij} A^\alpha_{ji}
}{
  \hbar(\omega_{ij}+\overline\omega_\alpha)
}
\\
&\begin{multlined}
\mathllap{\Delta_2 B(t)}
=
\sum_{\alpha\beta} 
F_\alpha F_\beta e^{-i(\omega_\alpha+\omega_\beta)t}
\sum_{ijk}
\Bigg(
\frac{
  f_{ik} B_{ij} A^\alpha_{jk} A^{\beta}_{ki}
}{
  \hbar^2
  (\omega_{ki}-\overline\omega_\beta)
  (\omega_{ji}-\overline\omega_\alpha-\overline\omega_\beta)
}
\\
+
\frac{
  f_{jk} B_{ij} A^\beta_{jk} A^{\alpha}_{ki}
}{
  \hbar^2
  (\omega_{jk}-\overline\omega_{\beta})
  (\omega_{ji}-\overline\omega_\alpha-\overline\omega_\beta)
}
\Bigg)
\end{multlined}
\end{align}
\end{subequations}

\end{document}

Notice that you can't place & inside multlined, so in order to align on the equal sign, you'll have to pretend that \Delta_2 B(t) has zero width. Hence \mathllap (again, from mathtools).

The result looks fine to me:

Answer 2

Exploiting the fact that the two left-hand sides have the same width, you can use aligned for the second equation:

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{fouriernc}
\begin{document}

\begin{subequations}
\begin{align}
&\Delta_1B(t)
=
\sum_{\alpha}
F_\alpha e^{-i\omega_\alpha t}
\sum_{ij}
-\frac{
  f_{ij} B_{ij} A^\alpha_{ji}
}{
  \hbar(\omega_{ij}+\overline\omega_\alpha)
}
\\
&\begin{aligned}
\Delta_2 B(t)
=
\sum_{\alpha\beta} 
F_\alpha F_\beta e^{-i(\omega_\alpha+\omega_\beta)t}
\sum_{ijk}
\Biggl(
\frac{
  f_{ik} B_{ij} A^\alpha_{jk} A^{\beta}_{ki}
}{
  \hbar^2
  (\omega_{ki}-\overline\omega_\beta)
  (\omega_{ji}-\overline\omega_\alpha-\overline\omega_\beta)
}
\\
+
\frac{
  f_{jk} B_{ij} A^\beta_{jk} A^{\alpha}_{ki}
}{
  \hbar^2
  (\omega_{jk}-\overline\omega_{\beta})
  (\omega_{ji}-\overline\omega_\alpha-\overline\omega_\beta)
}
\Biggr)
\end{aligned}
\end{align}
\end{subequations}

\end{document}

Note that \Bigg is not the right command; you should distinguish between \Biggl and \Biggr, in order that the spacings are right.