usepackage{graphicx}
usepackage{lipsum}

usepackage[pages=1,2]{background}

backgroundsetup
{scale=1.0, color=black, opacity=1.0, angle=0,
contents={
checkoddpage
         if
            includegraphics[width=paperwidth,height=paperheight]{1.pdf}
         else
            includegraphics[width=paperwidth,height=paperheight]{2.pdf}
        fi}
}

if page = 1
    includegraphics{...}
else
    includegraphics{...}