Thanks for the input @Pandal.
I did some digging, searching and merging and this seems fine to attach both invoice and delivery slip in a mail;
$context = Context::getContext();
$pdf_inv = new PDF($order->getInvoicesCollection(), PDF::TEMPLATE_INVOICE, $context->smarty);
$attachment_invoice['content'] = $pdf_inv->render(false);
$attachment_invoice['name'] = Configuration::get('PS_INVOICE_PREFIX', (int)$order->id_lang, null, $order->id_shop).sprintf('%06d', $order->invoice_number).'.pdf';
$attachment_invoice['mime'] = 'application/pdf';
$pdf_del = new PDF($order->getInvoicesCollection(), PDF::TEMPLATE_DELIVERY_SLIP, $context->smarty);
$attachement_delivery['content'] = $pdf_del->render(false);
$attachement_delivery['name'] = Configuration::get('PS_DELIVERY_PREFIX', (int)$order->id_lang, null, $order->id_shop).sprintf('%06d', $order->delivery_number).'.pdf';
$attachement_delivery['mime'] = 'application/pdf';
$attachement_inv_del = array($attachment_invoice, $attachement_delivery);
$templateVars = array(
'{firstname}' => $customer->firstname,
'{lastname}' => $customer->lastname,
'{id_order}' => $order->id,
'{order_name}' => $order->getUniqReference(),
'{invoice_number}' => Configuration::get('PS_INVOICE_PREFIX', (int) $order->id_lang, null, $order->id_shop) . sprintf('%06d', $order->invoice_number),
'{delivery_number}' => Configuration::get('PS_DELIVERY_PREFIX', (int) $order->id_lang, null, $order->id_shop) . sprintf('%06d', $order->delivery_number),
);
if (Tools::isSubmit('submitSendInvoice')) {
Mail::Send(
(int)$order->id_lang,
'order_conf',
Mail::l('Invoice for your order',
(int)$order->id_lang),
$templateVars,
$customer->email,
$customer->firstname.' '.$customer->lastname,
null,
null,
$attachement_inv_del,
null,
_PS_MAIL_DIR_,
true,
(int)$order->id_shop
);
}