I have been asking myself the same question for a long time, and finally came up with the idea of inserting a star symbol inside the main equation numbers, as explained in this thread. I have not yet had the opportunity to submit a paper using this technique, but guess that typical journals would accept it.
I believe you're using a two column format.
The environment split only accepts one alignment point; you can use \mathmakebox from mathtools (that also loads amsmath) and calc:
\documentclass[twocolumn]{article}
\usepackage{mathtools}
\usepackage{calc}
\begin{document}
\begin{align}
\begin{split}
z_1^{i,j}
& = a_1^{} x_1^{i,j} + b_1^{} \delta_1^{i,j} \\
& \qquad+ \sum_k y_k^{i,j}
\qquad \forall i,j
\end{split}
\\
\begin{split}
z_2^{i,j}
& = a_2^{} x_2^{i,j} + b_2^{} \delta_2^{i,j} + c_2^{} \\
& \qquad + \mathmakebox[\widthof{$\displaystyle\sum_k y_k^{i,j}$}][l]{z_1^{i,j}}
\qquad \forall i,j
\end{split}
\end{align}
\end{document}
Note that \, between factors is not used, while ^{} is recommended in order to push subscripts at the same level. The + on the second line shouldn't be under the equals sign, as it belongs to the right hand side of the expression.
However, the subscripts are not really level, because of j at the exponent, so you might want a slightly more complex adjustment:
\documentclass[twocolumn]{article}
\usepackage{mathtools}
\usepackage{calc}
\newcommand{\adjs}[1]{^{\vphantom{#1}}}
\begin{document}
\begin{align}
\begin{split}
z_1^{i,j}
& = a_1\adjs{j} x_1^{i,j} + b_1\adjs{j} \delta_1^{i,j} \\
& \qquad+ \sum_k y_k^{i,j}
\qquad \forall i,j
\end{split}
\\
\begin{split}
z_2^{i,j}
& = a_2\adjs{j} x_2^{i,j} + b_2\adjs{j} \delta_2^{i,j} + c_2\adjs{j} \\
& \qquad + \mathmakebox[\widthof{$\displaystyle\sum_k y_k^{i,j}$}][l]{z_1^{i,j}}
\qquad \forall i,j
\end{split}
\end{align}
\end{document}
Best Answer
Based on How to create a squiggle arrow with some text on it in TikZ?, I have come up with this:
Here is an example:
It works reasonably well, but is a bit ugly for "a"...