When working the same body part twice per week, it's important not to overdo it to the point that you can't complete the second workout of the week. This is how many newbies fall by the wayside: they go into beast mode on Monday, find themselves practically unable to raise their arms enough to blow their nose on Tuesday, tell themselves they need a rest day on Wednesday, and by Thursday their grand workout plans are a distant memory.
At the same time, muscle soreness isn't an excuse to back off. Don't be a baby. Develop the mental and physical toughness to constantly push yourself, and there's nothing you can't do!