Multiple Dual Currencies Settings
Some aspects of Accounting are controlled by custom settings. From Setup, click Develop | Custom Settings | Accounting Settings | Multiple Dual Currencies. You must be signed in with System Administrator permissions to perform these tasks. See the Salesforce Help for information on how to maintain custom settings.
Enabling this custom setting allows you to set up more than one accountancy currency as a dual currency.
This custom setting is disabled by default when you upgrade. This does not cause any issues if your org already has more than one dual currency set up, but you must enable this custom setting if you want to set up another currency as dual.