Here's everything you need to know about feeding your turtle:
http://www.redearslider.com/index_nutrition.html
Read every bit of it and then come back with your questions.
On your specific question in this post, it doesn't matter so much whether you feed your turtles once a day or twice, just so you don't feed too much in total. The rule for turtles under 4" is to feed them as much in pellets as would fit in their head (just the head, not including the neck) if it were hollow every day. If you want to break this up into two small servings that's fine, just so the total doesn't go over the headful-per-day amount.
When the turtle is over 4", you cut back to 1/2 headfull a day. Again, you can break this up into several small servings if you (and your turtle!) wish. Or, you can feed him every other day, a headfull at a time.
My turtle is over 4" so he's only entitled to a 1/2 headfull a day.
On the veggies, they can have about all they want of lettuce (green or red leafy type). On things like carrots, bell peppers, blueberries, steamed sweet potato, tomato, etc, a couple of servings a week is about right. Once a month or so they can have a high protein treat like a few minnows or some freeze dried crickets.