Hmm, this is a good question actually. We're using their API tickers, which provide trading volumes separately for different currencies. I assume they do calculations right and not provide doubled volumes through their API, however I couldn't find this explicitly stated anywhere. I'll approach them directly and find out. Will update you when I'll know more.
I'd naturally expect that summing the currency volumes would double-count the cross-currency trades. Will be interested to hear what you find out.