class foo1(Model):
     id = PrimaryKeyField()
     bar = TextField(null=True)

 class foo2(Model):
     id = PrimaryKeyField()
     bar2 = TextField(null=True)

 class Api()

     def get_foo1_by_id(id)
          return foo1.get(foo1.id == id)

     def get_foo2_by_id(id)
          return foo2.get(foo1.id == id)

     def save_changes()
          ...

foo1 = Api.get_foo1_by_id(1)

foo2 = Api.get_foo2_by_id(1)

foo1.bar = 'str'

foo2.bar = 'str'

foo1.save()

foo2.save()