Every currency has onshore and offshore rates. For example, you can buy dollars or Eurodollars, which is simple US currency kept offshore.

Eurodollars have different exchange rates because they don't have the reserve requirements from the Fed. i.e: it's less regulated.

The RMB has a heavier regulation whereby the exchange rate isn't allowed to fluctuate. This does increase the difference in value for the onshore and offshore currencies.

But having that distinction does not by itself make it a currency manipulator.

The main point behind the two currency system is to prevent money laundering. You can only convert offshore to onshore via state run banks. This also has the side effect of making it easier to control import and export flow thereby inflation.

The comment below about separate interest rates has nothing to do with this discussion.