Aligning equations while using cases in align environment

The \Centerstack retains a settable but fixed baselineskip. Thus, using \Centerstacks to set both the equations of the case, as well as (separately) the conditions of the case, will insure that their baselineskips are compatible.

\documentclass{report}
\usepackage{amsmath}
\usepackage[usestackEOL]{stackengine}
\stackMath
\setstackgap{L}{1.2\normalbaselineskip}
\begin{document}
\begin{align}
& a+b+c+d+e+f = 15000 && \text{for } a = 1 ... 15  \\
& D_t= \left\{\Centerstack[l]{\text{equation} ,\\ \text{another equation}}\right.&&
   \Centerstack{ \text{for } t = 1 ... 10 \\ \text{for } t = 1 ... 10}
\end{align}
\end{document}

Here are two ways to obtain it:

\documentclass{article}
\usepackage{mathtools}

\begin{document}

\begin{alignat}{2}
& a+b+c+d+e+f = 15000 &\quad & \text{for } a = 1 ... 15 \\
& D_t=
\begin{cases}
   \text{equation} , \\
   \text{another equation} ,
\end{cases} & & \begin{aligned}
   \text{for } t = 1 ... 10 \\\text{for } t = 1 ... 10
\end{aligned}
\end{alignat}

\begin{alignat}{2}
& a+b+c+d+e+f = 15000 &\quad & \text{for } a = 1 ... 15 \\
& D_t=
\mathrlap{\begin{cases}
   \text{equation} , & \text{for } t = 1 ... 10 \\
   \text{another equation} ,\hspace{1.9em}& \text{for }t = 1 ... 10
\end{cases}}
\end{alignat}

\end{document}