How to enlarge figures to take maximum available space?

There is no need to use minipage.

documentclass{article}
usepackage{graphicx}

usepackage{showframe,lipsum} % just for the example

begin{document}

lipsum[1][1-3]

begin{figure}[htp]

includegraphics[width=0.49textwidth]{example-image-a}hfill
includegraphics[width=0.49textwidth]{example-image-b}

caption{Two side by side figures filling the space, with some separation}

end{figure}

lipsum[1][4-5]
begin{figure}[htp]

includegraphics[width=0.5textwidth]{example-image-a}%
includegraphics[width=0.5textwidth]{example-image-b}

caption{Two side by side figures filling the space, with no separation}

end{figure}

lipsum[1][6-7]

end{document}

Using max textwidth with minor space between the figures

begin{figure}[h]
    begin{minipage}{0.5textwidth}
        includegraphics[width=textwidth]{image-b}
    end{minipage}
    begin{minipage}{0.5textwidth}
        includegraphics[width=textwidth]{image-a}
    end{minipage}%
end{figure}

If no space required between the figures put a % after the first minipage

begin{figure}[h]
    begin{minipage}{0.5textwidth}
        includegraphics[width=textwidth]{image-b}
    end{minipage}%
    begin{minipage}{0.5textwidth}
        includegraphics[width=textwidth]{image-a}
    end{minipage}%
end{figure}

edit --to prevent protruding image in to margin--with respect to suggestion by @mico

begin{figure}[h]
    begin{minipage}{0.4975textwidth}
        includegraphics[width=textwidth]{image-b}
    end{minipage}hfill
    begin{minipage}{0.4975textwidth}
        includegraphics[width=textwidth]{image-a}
    end{minipage}%
end{figure}