You can include context in your xml to difference field that triggering event

field name="a" on_change="onchange_intfield(a)"  context="{'somecode':'a'}"
field name="b" on_change="onchange_intfield(b)" context="{'somecode':'b'}"

And detect somecode value in your py def

def onchange_intfield(self, cr, uid, ids, number, context=None): 
         somecode = context.get('somecode',0)
         if somecode=='a':  
                 if number>8: 
                 return {'value': {'a': 8}}
         if somecode=='b':  
                 if number>8: 
                 return {'value': {'b': 8}}
          retun {}