From the wiki: "At any time, there can be only one wandering trader naturally spawned in the world."

My friend and I were in the world I generated for getting the achievements in the Village and Pillage update yesterday. I was tending crops while he was trading for the Master Trader achievement, when he told me that there were two Wandering Traders in the village at the same time. This is not supposed to be able to happen, so I went to see for myself, and I took a video clip:

https://www.youtube.com/watch?v=8weKjC7Su0s

This is a pure survival world. Cheats are not enabled, so there are no spawn eggs or summon commands. This was a pure fluke of game code.