You can do both parks in one day assuming that they are not crowded and you arrive when they open. When I visited US/IOA in September (post-Labor Day), I was able to do both parks in one day. All of the rides were walk ons.
As for taking a day out of Disney, I have no advice to offer. It's a matter of personal preference.
You can do the highlights of both parks in one day even if you go on a relatively busy day. Check out the website to see which rides you want to do and get there early.
You can do the highlights of both parks in one day even if you go on a relatively busy day. Check out the website to see which rides you want to do and get there early.