Shaving w/ a shavette, though similar to a straight, is still not quite the same thing. It's good for familiarizing yourself with the techniques necessary to successfully straight shave. If you're getting razor burn, then it's likely that you're going too heavy on the pressure. Razor burn is really a type of abraision, where you've removed too much skin.

As for one handed vs two handed shaving, you're correct in that you can go one handed w/ a DE, as the safety bar will do the stretching for you. With a SR, you MUST stretch the skin, or you will get cut, nor will you get much of a shave.

The holy trinity, if you will, for successful straight shaving is light pressure (almost none), proper angle (30 degreees for WTG, 15 for XTG, 5 for ATG), and stretching the skin. If you stick to those, you'll end up w/ a decent shave and minimize damage to yourself.