Experiment with django-money to have multiple money fields that share the single currency. May contain bugs, use with extreme caution.

fields.py

from __future__ import absolute_import, unicode_literals

from djmoney.models.fields import MoneyField, MoneyFieldProxy


class CurrencyLockedMoneyFieldProxy(MoneyFieldProxy):
    def __init__(self, field, currency_field_name=None):
        super(CurrencyLockedMoneyFieldProxy, self).__init__(field)
        if currency_field_name is not None:
            self.currency_field_name = currency_field_name

    def __set__(self, obj, value):
        obj_curr = obj.__dict__[self.currency_field_name]
        val_curr = str(value.currency)
        if obj_curr is None:
            obj_curr = str(self.field.default_currency)

        if obj_curr is not None and obj_curr != val_curr:
            raise ValueError(
                "Incompatible currencies: %s vs %s" % (val_curr, obj_curr)
            )

        return super(CurrencyLockedMoneyFieldProxy, self).__set__(obj, value)


class MoneyAmountField(MoneyField):
    def __init__(self, *args, **kwargs):
        self.currency_field_name = kwargs.pop("currency_field_name", None)
        super(MoneyAmountField, self).__init__(*args, **kwargs)

    def contribute_to_class(self, cls, name):
        proxy = CurrencyLockedMoneyFieldProxy(self, self.currency_field_name)
        super(MoneyField, self).contribute_to_class(cls, name)
        setattr(cls, self.name, proxy)

    def deconstruct(self):
        name, path, args, kwargs = super(MoneyAmountField, self).deconstruct()
        kwargs["currency_field_name"] = self.currency_field_name
        return name, path, args, kwargs

   # Django <=1.6: Haven't checked if South would work without changes or not.