Could be, but @mxchive is used for both deposits and withdrawals. Besides, I can't find any transactions between @mxchive and another possible MXC wallet. I also searched for other accounts with 'mxc' in the accountname, but could not found any other wallet.

Finally, I looked at a list of the largest transfers per day I don't see any large HIVE transfers to a possible other MXC wallet.

I think this is their only wallet.