Tian-Xian: It looks like you not can join a new tournametnt until the other tournament is finished. This is a copy/paste from the rules:

2.1) Number of joined tournaments

A free membership allows a player to join only one tournament and until this tournament is finished, the player cannot sign up for another one.

Tian-Xian: If you have finished all your games in a tournament and if you will not advance to the next round, then you can ask Fencer nicely and he will usually let you out of that tournament so you can enter another.