You can’t. There are (at least) two messages you get when trying to.

I bumume this one is for trying to do multiple threads at one time (in the same forum or not):

One message Posted:

It looks like you’re trying to enter a thread while you’re already in one. Please avoid using multiple browswer windows or tabs when in battle.

This is what you get trying to enter a thread in the first forum if you then open another one:

Another message Posted:

The thread you tried to enter is missing. Please avoid using multiple browswer windows or tabs when in battle.