If I qualify expert on both the M4 and M9 do I wear 2 devices or one with 2 bars (1x Carbine & 1x Pistol) on my blues?

It great to tell them to look it up but I would at least tell them which reference to look in.

I wore an expert badge with the PISTOL and AUTO RIFLE bars and a sharpshooter badge with the RIFLE bar. I had the same two badges for my entire service. The RIFLE and AUTO RIFLE qualifications were from 1968 and were still valid in 1984 because I never had to fire either rifle or auto rifle qualification again. I haven't checked the reg lately, but back then the marksmanship badge remained valid until you either fired that course of fire again or failed to fire the course of fire when required. It almost got sticky in 1983 while I was assigned to a Readiness Group where we didn't have weapons. By the reg, if your position wasn't assigned an individual weapon you would qualify with the standard service rifle. Fortunately we didn't have any weapons at all, so the policy was that we would qualify whenever the unit we were supporting did. That gave us armor guys a way to qualify with the pistol instead, and the unit cut our orders that way.

I qualified with the pistol more-or-less annually ("more-or-less" because some units were damn sloppy about scheduling qualifications).