To be honest, with the D4 I've stopped taking JPG photos. The camera writes so darn fast to both XQD and CF cards, and has such a large cache, that even with my need for bursts of shots when shooting wildlife and sports, JPG is unneeded.
I do have an exception. When shooting sports or news events as a stringer, and needing to send out my best photos quickly for almost instant publication, JPG is necessary. Also, if doing a charity event, where they're doing fundraising via souvenir portraits being done. I use RAW (for stock) and JPEG for the instant publication for sports and news. I use straight JPEG for the fundraiser portraits, and the card goes right into the printer for the take home prints, or some places provide a memory stick on which we put the photos for each person.
I'm pretty adept with RAW processing and can knock out the photos pretty quickly in Photoshop, so for sharing a few shots, I can get them done okay.
So, I'm normally using overflow mode with the D4.
As to the setup with the D800, it works the same way.
Frankly, in both these cameras, unless you're shooting at their highest speed of frames per second, writing to the memory cards shouldn't be a problem, unless you have older slow cards in the slots.
(I'm not so sure that Photo Mechanic ingests to the primary and secondary drives simultaneously, even with the new version. It seems to me I see the lights on my external hard drives going back and forth. They don't seem to be writing to the drives simultaneously.