1) create fields like:

'product_qty': fields.float(string='Product Qty'),

'planned_qty': fields.float(string='Planned Quantity'),

2)define in xml

<field name="planned_qty" on_change="onchange_planned_qty(planned_qty)"/>
<field name="product_qty" on_change="onchange_product_qty(product_qty)"/>

3) call onchange function

def onchange_product_qty(self, cr, uid, ids, product_qty, context=None):
        if product_qty:
            return {'value': {''planned_qty': product_qty}}  

      else:
            return {'value':{''planned_qty': ''}}

    def onchange_'planned_qty(self, cr, uid, ids, 'planned_qty, context=None):
        if planned_qty:
            return {'value': {'product_qty': planned_qty}} 

       else:
            return {'value': {'product_qty': ''}}

In odoo 14, try "post_init_hook"