Adding another line

Hi. Since I won't be eligible to get a full subsidy on the iPhone 4 till 2/20/11. (got the 3GS on partial discount $499 on launch). I was wondering how it would work out if I tried to add another line?

Do the lines share minutes? Not sure, I am the sole person on this line. If I wanted to cancel the primary line or the one I open, how would this go? How soon could I do it?

Also, would I have to pay an activation fee of sorts? What other fees are out there?

Thanks!