a lazy approach would be to put twice <div class="page"> tag on your report template. this way you'll have double content , one header, footer.

try these code on template

put one more doc_id in  foreach loop like the below example

<template id="invoice_sale_report">

<t t-call="report.html_container">
<t t-foreach="[doc_ids,doc_ids]" t-as="doc_id">
<t t-raw="translate_doc(doc_id, doc_model, 'partner_id.lang','report.sale_invoice')" />
</t>
</t>
</template>