pos/pos_backend_partner/views/assets.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <template id="assets" name="pos partner" inherit_id="point_of_sale.assets">
        <xpath expr="." position="inside">
            <script type="text/javascript" src="/pos_backend_partner/static/src/js/pos_partner.js"></script>
        </xpath>
    </template>

    <template id="assets_backend" name="back partner " inherit_id="web.assets_backend">
        <xpath expr="." position="inside">  <!-- both back and POS -->
            <script type="text/javascript" src="/pos_backend_partner/static/src/js/back_partner.js"></script>

            <link rel="stylesheet" href="/pos_backend_partner/static/src/css/pos_backend_partner.css"/>

        </xpath>
    </template>

</odoo>