+1 Odoo Honduras, thanks

you can't do it using xml, you need to do it using the new api style for onchange, like:

    @api.onchange('marital_status')
    def onchange_m(self):
        self.sex = 'm'
    @api.onchange('marital_status')
    def onchange_f(self):
        self.sex = 'f'

There is no way to do it, but there is no need to do 2 onchanges, you can do it once, and return more values, it depends on what you are trying to do.