Footnote span over two columns without number and long "break line"

Update The dblfloatfix package allows two-column floats to go to the bottom of the page. WIth this you can just do:

begin{figure*}[b]
hrule
begin{equation}
E=mc^2
end{equation}
end{figure*}

Note that you may need to put the figure* environment earlier in the document than you think for it to be placed correctly.

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Old solution that doesn't require external packages

LaTeX really doesn't do multiple columns well and this is one of those cases that we bump into its limitations. My first thought would be to do this in a figure* and put the rule in manually but LaTeX can't do two-column figures anywhere but the top of the page.

An ugly way to do this would be to do

begin{figure}[b]
begin{minipage}{textwidth}
hrule
begin{equation}
E=mc^2
end{equation}
end{minipage}
hss % Allows minipage to stretch across both columns without complaint
end{figure}

and then in the second column

begin{figure}[b]
vspace*{5baselineskip} % or however many lines you need to clear for the equation
end{figure}

Note that this will end up being fragile in that small changes to the document could cause the figure placement to get messed up moving things where you don't want them. Maybe there's some improvement in the last 25 years of LaTeX packages I don't know about.