there are two train tracks on the RER B CDG platform, do they both go to Paris? Does the line start at CDG?
Yes, CDG 2 (the Terminal 2 train station) is the start/terminus of the RER B line. (There's another branch of the RER B line that terminates at Mitry-Claye, but if you're at CDG 2, you're at the start/end of the line for this branch of the RER B line.)
Yes, both tracks go to Paris. One will leave before the other when there are two trains at the platform at the same time.
If you're at Terminal 1 train station (not actually inside Terminal 1, but inside a building called Roissypole and is the closest train station for Terminal 1 and Terminal 3), one of the tracks is going towards CDG Terminal 2 / terminus and the other track is going towards Paris.