you can reset doc variable like first called function. work for me
$(".button").click(function(){ var doc = new jsPDF('landscape'); doc.addImage(frontImg, 'PNG', 0, 0, 300, 210); doc.addPage('a6','l'); doc.addImage(backImg, 'PNG', 0, 0, 300, 210); doc.save('file.pdf');
doc = new jsPDF('landscape'); });

try to initialise the doc once again...

$(".button").click(function(){ var doc = new jsPDF('landscape');
doc.addImage(frontImg, 'PNG', 0, 0, 300, 210); doc.addPage('a6','l'); doc.addImage(backImg, 'PNG', 0, 0, 300, 210); doc.save('file.pdf'); doc = new jsPDF('landscape'); });