You can use the following method for implementing on_change method multiple fields with just one function.The fields must be in the same model.

in xml:

#in xml
<field name='test' on_change="get_func(test,'string')"/>
#and in python use 
@api.multi
def get_func(self,value,field):
     ""here you can compare the field name and exec the desired lines using the if """

@api.onchange('staff_fname', 'staff_lname')

def funa(self):

if self.staff_fname and self.staff_lname:

s=(self.staff_fname or '') + (self.staff_lname or '')

self.staff_name = s.strip()

In this onchange function is execute when first anem and last anme are specified in your view.