There is NO device limit on an itunes account, you can have unlimited ipads, iphones, pods whatever all on the same itunes account. The only limit is a max of 5 computers can share one itunes account. Any devices that share the same account have access to all the apps that they are capable of running with no additional fee.
I'm currently sharing 5 ipods, 2 iphones, an ipod touch and an ipad on my account. Soon to add a second ipad.
The only exception might be the subscription for AT&T navigator app, because there is a 9.99 monthly charge by AT&T for it. But I only use it on my iphone, not my husbands so I'm not totally sure. Haven't tested putting it on his iphone. But since it's itemized on my iphones bill, I think you pay per phone.
As for how do you set it up, you sync the device with it's USB cable to your computer and register it to your itunes account, then sync it like usual. It will sync whatever you tell it to sync. It doesn't automatically sync what's on your other devices as each one is recognized and has it's own backup. You can also just download apps you already own on that account, directly on the device using the ipad app store application.
You can tell it to restore your new ipad from another backup if you want to, it will ask when you register it. That would load whatever content was sync'd on your last backup. I haven't tried it from an ipad backup, as I only have one right now, but it asked me if I wanted to restore my present ipad using my iphone backup ( which I did) when I first got it. So it probably will ask that with a second ipad also.