Below are the sentences 1) from scientific Journal 2) from a book. Since both are published text, however, my confined knowledge makes me ask this question. In 1) sentence two things are mentioned "Intermittence" and "variability", still the auxiliary verb "is" used why so? In the same way in sentence 2) "stopping" and "allowing" are used, while the verb "allows" is used which should be "allow". Can you please help me in this regard,
1)Intermittence and variability of renewable resources is often a barrier to their large scale integration into power systems.
2)stopping and thinking allows us to gain perspective on both the successes and failures of our day so that we can find the lessons within them.