Hi,

You can achieve this using onchange function, or by making a compute field or by a related field.

Using  a relate d field will be easier here.

Sample:

field_1 = fields.Char(string="Test")

field_2 = fields.Char(string="Test 2", related='field_1')

For Reference

1. How To Add Related Field In Odoo
2. How To Add Onchange Function in Odoo
3. How To Add Compute Field In Odoo
   

Thanks