Why, when you have a multiple page document, do the lines from the second page show up on the first page and how do i prevent that from happening?

Sounds like you have one set as a background page and the other/s are using that background.  Have a look at page setup and this link if that's the case:

http://office.microsoft.com/en-gb/visio-help/create-a-background-page-HP085050933.aspx

Hope the helps.

Best regards

John

Unfortunately thats not it. They were both set as foregrounds. I tried flipping them and playing around a bit with it, but still came across the same problem.

Sounds like a video drivers problem. Ensure you have the latest video drivers for your computer installed, and in the display properties turn off (or just reduce) any video hardware acceleration.