Hi, 

We can add a many2many field inside mail.activity 

class MailActivity(models.Model):
    _inherit = 'mail.activity'
 
    multiple_users = fields.Many2many('res.users', store=True)
 
#Add this function inside 
    @api.model
    def create(self, vals):
        assigned_user_id = vals['user_id']
        assigned_user_name = self.env['res.users'].browse(vals['user_id']).name
        vals['display_names'] = assigned_user_name
        res = super(MailActivity, self).create(vals)
        if 'multiple_users' in vals and vals['multiple_users']:
            related_user_ids = self.env['res.users'].search([('id', 'in', vals['multiple_users'][0][2])], order='id')
            for each in related_user_ids:
            if each.name != assigned_user_name:
                vals['user_id'] = each.id
                vals['display_names'] = each.name
                res = super(MailActivity, self).create(vals)
        return res

And add that to the view.

<record id="mail_activity_view_form_popup_inherit" model="ir.ui.view">

    <field name="name">mail.activity.form.inherit.calendar</field>

        <field name="model">mail.activity</field>

        <field name="inherit_id" ref="mail.mail_activity_view_form_popup"/>

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

    <xpath expr="//field[@name='user_id']" position="after">

                <field name="multiple_users" widget="many2many_tags" string="Additional Assignees"/>

        </xpath>

        </field>

</record>

So here we can add separate activity for each users we select in the multiple_users field.

Regards