# Equations tag placement: In between two equations

I would like to use only one tag (O.C.) for these two equations and place it on the right side as by default, but, (vertically), in the middle between the two equations.

Is this possible? Is this right according to LaTeX rules?

\documentclass[12pt, a4]{article}
\usepackage{amsmath,amsfonts,amssymb}

\begin{enumerate}[a.]
\item \textit{Solve for the equilibrium prices of the contingent claims and the resulting consumption amounts.} Knowing already the first derivative of the two utility functions, we set the \textit{Optimally Condition}
\begin{gather}
\tag{O.C.} \frac{0.5}{\Pi_{NL}} \cdot \frac{1}{C^A_{NL}} = \frac{0.5}{\Pi_{L}} \cdot \frac{1}{C^A_{L}} \quad \to \quad \Pi_{NL}\cdot C^A_{NL} = \Pi_{L} \cdot C^A_L \\
\tag{O.C.} \frac{0.5}{\Pi_{NL}} \cdot \frac{1}{C^B_{NL}} = \frac{0.5}{\Pi_{L}} \cdot \frac{1}{C^B_{L}} \quad \to \quad \Pi_{NL}\cdot C^B_{NL} = \Pi_{L} \cdot C^B_L
\end{gather}
\end{enumerate}
\end{document}
• Aside: For correct English, the expression should be \textit{Optimality Condition}, not \textit{Optimally Condition}. (Optimally is an adverb, optimality is a noun.) – Mico Oct 25 '16 at 3:41

$$\tag{O.C.} \begin{split} \frac{0.5}{\Pi_{NL}} \cdot \frac{1}{C^A_{NL}} &= \frac{0.5}{\Pi_{L}} \cdot \frac{1}{C^A_{L}} \quad \to \quad \Pi_{NL}\cdot C^A_{NL} = \Pi_{L} \cdot C^A_L \\ \frac{0.5}{\Pi_{NL}} \cdot \frac{1}{C^B_{NL}} &= \frac{0.5}{\Pi_{L}} \cdot \frac{1}{C^B_{L}} \quad \to \quad \Pi_{NL}\cdot C^B_{NL} = \Pi_{L} \cdot C^B_L \end{split}$$