project-tools/project_task_assignees_avatar/views/task_kanban_avatar.xml

<?xml version="1.0" encoding="UTF-8"?>
<odoo>
    <record id="view_task_assignees_avatar" model="ir.ui.view">
        <field name="name">view.task.assignees.avatar</field>
        <field name="model">project.task</field>
        <field name="inherit_id" ref="project.view_task_kanban" />
        <field name="arch" type="xml">
            <xpath expr="//field[@widget='many2one_avatar_user']" position="after">
                <field name="first_assignee_id" widget="many2one_avatar_user"/>
            </xpath>
        </field>
    </record>
</odoo>