begin{equation}
    A=
    begin{cases}
      1, & text{if} x=1, x=2 \
      2, & text{else}
    end{cases}
end{equation}

begin{equation}
    A=
    begin{cases}
      1, & x=1 \
        & x=2 \
      2, & text{else}
    end{cases}
end{equation}

documentclass{article}
usepackage{amsmath}
begin{document}
begin{equation}
    A=
    begin{cases}
      1, & text{if           $x=1$}\
         & text{phantom{if }$x=2$}\
      2, & text{else}
    end{cases}
end{equation}
end{document}