Double up insurance??

Aly • Happily married. MC 4/2015👼🏻Baby girl 10/2016 💕 baby boy due 2/2018 💙
I have an insurance question for the mamas....
We are currently on my plan, and my husbands work has open enrollment in November. He is thinking we could also enroll in his and have both. Our premiums are relatively low because we have government jobs. It would cost is about $120 per month if we had both (mines $60). He wants to do this because he thinks then we will have less to pay out of pocket and can avoid reaching our deductible. 
However, I have an HSA account and a deductible of 2,000, with the HSA crediting us $1,500 and the insurance paying 90-10 after its kicked in. So the only way this makes sense to me is if our bills would be less than $500 out of pocket after both insurances paid out.
Our baby is due November 30, so I'm also worried that the pregnancy would be considered a pre-existing condition and his insurance wouldn't cover it anyway.
We've been fortunate with good health so we've never really had to deal too much with insurance before. I know babies have frequent check ups and our expectation of what we need for insurance will be changing soon.
Any insight would be appreciated!