Hulletttribe, no I'm not saying that to do a rally you have to have one of the tanks I mentioned. Not at all.
Those are just some of the tanks commonly used by LD riders whether on IBA certificate rides, the Iron Butt Rally or any other type of rally or just tootling across the countryside for the fun of it.
If you do choose to ride the beast with two backs in a rally, there will be opportunities beforehand to ask the rally staff if your tank complies with their rules. At the rally there will be a technical inspection as part of the scrutineering process.
As for the vent line vs vent cap..... this comes down to practicality and safety and may vary again with the ride you are planning. It is a different thing to just bolt on a second tank to the back of your bike and go riding vs bolt a second tank onto your bike and enter the Iron Butt Rally. One will not be inspected (you just riding) and the other will be inspected in light of rules (IBR). If the rules for the rally call for a vent line, you'll need a vent line. (Here's and old version
http://ironbutt.com/ibr/2013/IBR2013Rules.pdf)
I didn't mean for this thread to get bogged down in the requirements for the Iron Butt Rally.
What bike are you riding and what tank are you thinking of adding? There are hundreds of riders in the IBA who have added tanks of various types to their bikes, you may get some quality tips or advice if they know what you are planning.