XC helmet for road biking, or road helmet for XC? I'm not a very serious biker in either category, I need a new helmet and I'm thinking that I would rather spend $200 on one awesome helmet rather than $100 on two just-good helmets. 

I understand that an XC helmet will have more padding and a road helmet will lighter and more aerodynamic. One one side, I feel like added protection of the XC can't hurt, but then the aerodynamic thing comes into play. Also in favor of the road helmet, I don't think I ride crazy enough trails to merit the XC helmet. I mostly ride mild to moderate single tracks in the woods of New England. 

For a road helmet, I've been searching for a few hours and I've found that POC makes a road and XC helmet based on the same design called the Octal. In the road bike variant, it comes with MIPS. 

For an XC helmet, I've noticed a few models come with GoPro mounting systems with quick release in case of a collision. I do a lot of night time XC riding and I love the idea of putting a flashlight on that. The Bontrager Lithos and a few Lazer helmets have that feature. I'm not sure how odd they'll look or if the visor wind resistance will actually be a problem on the road bike. 

Are there any better middle ground helmets that I'm missing? Am I risking my safety more one way or the other? Should I change my mind and buy two instead of one? 

I've been riding with a Trek Vapor for over 10 years. Anything is an improvement!
There is significant debate over the statistical benefit of wearing a helmet or not. My guess is most of use wear a helmet just in case the "Helmets are safer' camp is right but wonder if they really are.  

I cannot imagine the difference between helmets from a safety perspective being statistically significant. "Road. XC etc are just marketing terms - the helmets meet standards, and these tell you how much protection you get. Much better to sell two helmets and make twice the profit, easy to do by making them look different enough that peer pressure has people buying a helmet for every task - I bet someone sells a "Commuter" helmet and a "Bike Courier" helmet somewhere. 

You have not mentioned the most important aspect of helmet purchase - fit. A helmet that fits you is significantly better than any stickers as to what standards it meets.  

Presuming then you are choosing from a range of helmets that meet published safety standards, and are happy with the manufacturer as far as their honesty around this (Many standards are declarations of conformance rather than independently tested), choose a helmet that fits first, and has the features you are after second.