You can buy a 1 day/two park ticket for around $70-$80. IMHO, buy the 2 day/2 park ticket for $15 more. Unused days never expire.
As for doing both parks in one day, I've always wanted to try it. If you want to do it, try doing this. You won't be able to see everything, but you will be able to do the big things.
1.) Arrive 1 hour before opening
2.) Go to IOA first, buy tickets, use the bathroom, etc.
3.) When the ropes drop, do the rides in this order:
Spider-Man
Fearfall
Hulk
Cat in the Hat
One Fish, etc. (if you really want to)
Dueling Dragons (Fire, then Ice -- no front seat rides)
JPRA
Ripsaw Falls
Popeye
4.) Go to USF
5.) Watch the queues. If a ride has a short wait (15 minutes or less), do it. If T2 has a short wait, do that. Otherwise, eat lunch and watch some shows. Perhaps even do MIB or Jaws in the single rider's line.
6.) At around 3 pm or 4 pm, take a look at the queues. The wait times will probably drop to 15-30 minute waits, except for perhaps MIB. Do the rides. I would start with ET and then do:
BTTF
MIB (single rider's line if you haven't done it)
Jaws
Earthquake
Kongfrontation
Twister
Hanna Barbera
T2 (if you haven't seen it)
If you have time, do the rides that you want to do again at USF or IOA, or watch some more shows.