Adding a combination of leave type in the TimeOff module in the LeaveForm? Odoo13 - odoo

Question

I needed to add a feature that can combine two leave types in the TimeOff module form, and when a user submits the form the leaves gets deducted from the respective time off type. (In the form I am thinking to add a second dropdown menu where he/she can add the other time off).
I am using Odoo13, I am inheriting the module, but can't figure it out to add a dropdown in which the user can select multiple leaves.
model=hr.leave, externalid = hr_holidays.hr_leave_view_form
Work that I have done:
I have created a module which inherits the hr_holidays module in Odoo13, In the leave form i have sucessfully added the dropdown which is above but it is selecting the same time-off type which is selected above.
Here is the xml file:
<?xml version='1.0' encoding='UTF-8' ?>
<odoo>
<!--Adding the Drop Down-->
<record id="hr_leave_view_form_addnewleave" model="ir.ui.view">
<field name="name">leave.addleave</field>
<field name="model">hr.leave</field>
<field name="inherit_id" ref="hr_holidays.hr_leave_view_form"/>
<field name="arch" type="xml">
<xpath expr="//div[#name='description']" position="before">
<div class="row" name="status_id">
<label class="col-2 mr-0" for="holiday_status_id"/>
<!-- RLI FIXME: we should avoid redefining the domain in the view when there already is a domain on the model -->
<field name="holiday_status_id" domain="['&', ('virtual_remaining_leaves', '>', 0), '|', ('allocation_type', 'in', ['fixed_allocation', 'no']),'&',('allocation_type', '=', 'fixed'), ('max_leaves', '>', '0')]" context="{'employee_id':employee_id, 'default_date_from':date_from}" options="{'no_create': True, 'no_open': True}" class="col-9 pl-0" nolabel="1"/>
</div>
</xpath>
</field>
</record>
</odoo>
In this the code with is for the dropdown.
The LeaveForm image with dropdowm
Thanks in advance.Please look into the photo that I uploaded.