Argthtjtr

I have seen some posts close to what I need, but not quite what I need.

I would like to produce two equations on the same line with one equation number adjusted to left and one to the right. Both equations should be multiline.

And I want to generate a tag for both equation labels.

So the output should look like what is produced by the code below.

begin{array}{llrr}

      &    a=b                  &      x=y    &   \

(eq1) &    c=d                  &      y=z    & (eq2)     \

      &    e=f                 &       u=v

end{array}

I propose this solution, with a code borrowed on this site from @DavidCarlisle:

documentclass{article}

usepackage[utf8]{inputenc}%

usepackage{tabularx} 

usepackage{mathtools, nccmath}



makeatletter

newcommand{leqnomode}{tagsleft@trueletveqno@@leqno}%

newcommand{reqnomode}{tagsleft@falseletveqno@@eqno}%

newcommand*{compress}{@minipagetrue}

makeatother



begin{document}



Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text.



{centeringcompress

begin{tabularx}{linewidth}{>{leqnomode}XX}

begin{equation}

begin{aligned}

       a & =b \

    c & =d \

       e & =f

end{aligned}

end{equation}

 &

 begin{equation}

begin{aligned}

x & =y \

 y & =z \

u & =v

end{aligned}

end{equation}

end{tabularx} vspace{-baselineskip}}



 Some more text. Some more text. Some more text. Some more text. Some more text. Some more text. Some more text. Some more text.



end{document} 

Like this:

documentclass{article}

usepackage{tabularx}

newcolumntype{C}{>{centeringarraybackslash}X}

usepackage{amsmath}



%---------------- show page layout. don't use in a real document!

usepackage{showframe}

renewcommandShowFrameLinethickness{0.15pt}

renewcommand*ShowFrameColor{color{red}}

%---------------------------------------------------------------%

usepackage{lipsum}



begin{document}

lipsum*[66]

    begin{center}

    begin{tabularx}{linewidth}{@{} lCCr @{}}

refstepcounter{equation}

(theequation)label{eq:left}

    &   $begin{aligned}

        a & = b \

        c & = d

        end{aligned}$

        &   $begin{aligned}

            x & = y \

            z & = w

            end{aligned}$

            &   refstepcounter{equation}

                (theequation)label{eq:right}

    end{tabularx}

    end{center}

dots, see eqref{eq:left} and eqref{eq:right} dots

lipsum*[66]

end{document}

Addendum: I figured out how to use my own tags for equations:

documentclass{article}



usepackage{amsmath}

makeatletter

newcommand{leqnomode}{tagsleft@true}

newcommand{reqnomode}{tagsleft@false}

makeatother



%---------------- show page layout. don't use in a real document!

usepackage{showframe}

renewcommandShowFrameLinethickness{0.15pt}

renewcommand*ShowFrameColor{color{red}}

%---------------------------------------------------------------%

usepackage{lipsum}



begin{document}

lipsum*[66]

    begin{center}

    vspace{-baselineskip}

    begin{minipage}{0.5linewidth}leqnomode

begin{equation}label{eq:left}

begin{split}

        a & = b \

        c & = d

end{split}tag{*}

end{equation}

end{minipage}begin{minipage}{0.5linewidth}

begin{equation}label{eq:right}

begin{split}

        x & = y \

        z & = uvw

end{split}tag{**}

end{equation}

    end{minipage}

    end{center}

dots, see eqref{eq:left} and eqref{eq:right} dots

lipsum*[66]

end{document}

I'm not sure it is a good idea.

documentclass{article}

usepackage{amsmath}

usepackage{array}

usepackage{lipsum} % for context



newenvironment{doublynumberedequation}[2]

 {%

  unskip

  refstepcounter{equation}label{#1}%

  refstepcounter{equation}label{#2}%

  defsecondlabel{#2}%

  begin{equation*}

  begin{tabular*}{displaywidth}{

    @{extracolsep{stretch{1}}}

    l

    >{$displaystyle}c<{$}

    @{extracolsep{stretch{2}}}

    >{$displaystyle}c<{$}

    @{extracolsep{stretch{1}}}

    r

    @{}

  }

  eqref{#1} &

 }

 {& eqref{secondlabel}end{tabular*}end{equation*}}



begin{document}



lipsum*[4]

begin{doublynumberedequation}{left}{right}% use two meaningful labels

  begin{aligned}

  a&=b \

  c&=d \

  e&=f

  end{aligned}

&

  begin{aligned}

  x&=y \

  y&=z \

  u&=v

  end{aligned}

end{doublynumberedequation}

lipsum[5]



end{document}