I have created custom mail templates in mail.template model as below

    <record id="record_id" model="mail.template">

    <field name="name">Template</field>

    <field name="email_from">dummy@mail.com</field>

    <field name="subject">${object.name}</field>

    <field name="partner_to">${object.partner_id.id}</field>

    <field name="model_id" ref="project.model_project_task"/>

    <field name="auto_delete" eval="True"/>

    <field name="body_html" type="xml">

issue is that if partner have multiple email addresses they are all delivered but the receivers only sees as they are the only receivers. 

Same thing was also mentioned in the following post

https://www.odoo.com/fr_FR/forum/aide-1/multiple-email-addresses-on-template-115544

i have tried these options but its not working as required

    <field name="email_to">${object.partner_id.id}</field>

    <field name="email_to">dummy@mail1.com, dummy@mail2.com</field>

    <field name="partner_to">${object.partner_id.id}</field>

    <field name="partner_to">dummy@mail1.com, dummy@mail2.com</field>