Hey,

I am adding a second bath to my house right behind the existing bath. I already have a 2'' vent pipe for the existing bath. That vent is up from the toilet and shower I am installing. It is my understanding that the vent has to be down for the toilet correct. How far down can it be? Does the vent have to come off the main line?

Unless you have a illegal "S" trap install on your lavatory the group vent should come off the lavatory. Let me show you a typical rough in. Most bathroom groups are roughed in like this.
Toilet connects to sewer main or the stack vent. Lavatory connects to toilet drain and runs a vent off the top the stubout tee out the roof or revents back into a dry vent in the attic.. The toilet wet vents through the lavatory vent and the tub/shower connects to the lavatory drain and is wet vented by it. This is a normal rough in and is acceptable both by local and state codes and also The Standard Plumbing Code Book in 90 percent of the country. Check your local codes.to make sure you're not in the excluded 10 percent.. The vent off the lavatory may be run out the roof or revented back into a dry vent in the attic or if you're reventing back into a fixtures dry vent you must make your connection at least 6 inches over that fixtures flood rim.
Good luck, Tom

Not to take anything away from speedball1's answer -- but page 53 should give you a better picture

Plumbing a House - Google Book Search

Note : ALL DRAINS ARE VENTED ----------------> If this helps -- Please rate my answer

And not to conflict with Biggsies answer but page 53 depicts a individual vent for every fixture. Unless area or local codes mandate a vent for each fixture a wet vent will give you one vent to worry about instead of three. Regards, Tom