don't do it!! its addicting and expensive!!! I am complaining to the BBB cause its too damn expensive!! j/k I am just starting out too just did my 2nd track day in NJ and already got bumped up. All I can tell you is follow the race line, proper body position and follow the race line ...everything else will com natural and also depends on your OH SHIT factor. But, enjoy and save money cause its going to cost you a lot of money!
if you have the $$ u're better off doing both. u're there, your relaxed and set up, and u spent the first day getting things down so you're ready to rail the second.
personally, i don't do 2 days at places i know already pretty well. it gets to be a waste of tires and money to just do stupid laps. i start to screw around and see how many times i can spook my friends and stuff but looking at them in the corners as they try and keep up.
but... if you have the cash try and do the 2 day everynow and then. but stay ahead of it by drinking water days prior and doing a lot of working out IN the heat to get your body prepared to handle the heat and retaining more water. you need a clear head and no heat related issues so you an focus on passing that schmuck who just won't get out of the way.