Depends on your mission, and your role in that mission.

Within the same vehicle it could be a good idea for some and a terrible idea for others.

As a driver, having your secondary chest mounted can save your life. (Try getting your primary into a firing position when someone in a crowd gets ready to toss a frag through your window) Same thing for the guy in the ring mount. It's not easy to spin the M2 into position to take a shot at the guy, but a chest mounted secondary can be just the ticket.

For the Corpsman in either rear seat, terrible idea.

I'd say if the secondary is in a position where it is easier to employ in the position you most often find yourself in, then great.

If you're ever in a position where you may find yourself leaning over an enemy combatant, I'd never want that thing anywhere in their reach. (Been there)

That doesn't just go for Corpsmen and Medics. If you're balling up a bad guy and have a knee in his chest or you're taking him to the ground, a secondary on your chest will look very attractive, especially when you have both of your hands full prepping flex cuffs and controlling him. Even if you're working with another shooter providing cover, shit can happen fast. Keep it on your hip.

Just my thoughts.