I'm a skinny guy too and I definitely don't cut the circuits out. If you're doing 2 circuit days a week, that means you're on month 1 which is for beginners. If you're a beginner, I'd be willing to bet you would greatly benefit from doing the circuits because I'm a beginner too and they really helped to increase my cardio/endurance, which lets me bring way more intensity to the PPL days.
As one example, trying to push myself on squats used to be very hard. I'd have to stop lifting long before my muscles would fail due to my poor cardio. After doing circuit-a for a few weeks, my conditioning improved greatly. Now I can get a much better workout when I squat because I can push myself way harder than I used to.
Just make sure you increase your caloric intake to make up for the additional calories you burn.
Hope that helps.