There will be a potential difference between spots of each attachment. Resistance between the 2 points..EG rivet connection ,seams etc. The standard practice it to have 1 earth bar connection and all earths go back to that one place. The radio case earth and DC negative will come from the earth bar and any shield attached to the radio case eath like the intercom shields are left open ended so there is no second return path
The only problem I have had with my M760 is me not following the installation instructions to the letter and believing other people whose opinion I now know to be rubbish. I threw out the original harness I made, and made a new one with proper shielded cable, earthed at the radio end only, installed a power filter and ferrite chokes where specified as per the manual. Now everything works well. I still get a bit of noise when hearing broadcasts from long distance in CTAF but then these are 40 or 50 or even more miles away so it doesn't matter. This can be improved with replacing the mag switch cables with shielded cable & earthing these at the engine so I will probably do this one day when I am looking for another project.
My Microair also has electrical type interference when receiving distant stations however on a recent trip i did a transmit check with Melbourne Centre and they received me 5X5.
I find that the display is difficult to read in sunlight, and very difficult to change frequency in any sort of turbulence!
I also had problems trying to speak with a tech at Microair, they were not interested in discussing issues a couple of years ago.
I will replace mine with a Trig to match my Transponder when the time comes.