That's pretty easy to fix. For one thing, when China has to ship to the other country, that's automatically a tax on their profits. We don't get the money, but China loses it, so it's still a win for us.
Then, this 2nd country is going to want some sort of bribe from China, so that's a little less profit for China. Again, we don't get the money, but China loses it, so another win.
3rd, this other country that's working for China will have at least 10% tariff on it, so that's more money lost for China, and this time we DO get it, so it's win/win for us.
Finally, if this whole 2nd party shipping scheme turns out to still be profitable enough for China to do it, we WILL find out, and Trump will simply raise the tariff on the 2nd party country in order to make it no longer profitable to China and to punish that country. China may slip a few shipments by this way, but they won't get away with it for long, and any country who's willing to try it will probably get more punishment than profit.