I would not do them at the end of back day, even with lighter weights. The reason is that you will be tired, and risk compromising your form. Do them at the start of back day, after a proper warmup, and gradually build up to higher weights. Don't worry if you are not lifting much at the start - instead concentrate on proper form. Have someone who knows what he/she is doing watch you or post videos on this forum if you are unsure. Keep the reps per set low as well, as the more reps you do, the more your form will degenerate.
I don't think 2X/week is too much at least for now, as long as you keep them close to the start of your workout. As you progress to higher weights, you may find that once per week is enough to fully tax your system. Deadlifts are hard, but well worth the effort.