Originally Posted by milepig

All covered in the thread link I posted in greater detail. There is no such concept as "round trip" they are simply 2 tickets per person that don't expire. Just go ahead and buy four for the 2 of you and tuck the other 2 away for the return.

es, the RER B is your return. The trains marked for CDG and there are boards on the platform which indicate which trains are coming next. Be sure to get an express, the indicator board will show fewer stops lit.