In the ranked battle menù if you server got a merge, you should have two chart option, the first on the left for your original server and the second one on the right for the merged server chart. If you didn't get the merge yet, you should only see a unique chart.

I just noticed that there is another way to get the second free SS convoy. You have to either be in top 50 rank of the non merged server Rank Battle and in the top 100 of the Merged server Rank Battle or the opposite, top 100 rank of the non merged server Rank Battle and in the top 50 of the Merged server Rank Battle.

I have the second condition and I am able to do 2 SS convoy for free, correct me if I am wrong :)