1)In an old duplex a lot of existing circuit feeds are 12-3.I used one of them for kitchen
counter receptacles;the problem is when using a GFI tester both GFI'S pop.Is this usual, legal; is because of shared nuetral?
2)AFI similar problem-I think previous sparky
tied noodles in attic JB;result is that one or the other AFI will hold, but turning 2nd
AFI kicks off the first one; will shared nuetral do this ?

You can not share a neutral on the LOAD side of these devices. A fault is sensed thru the neutral, a split return path. If you are using a shared neutral on these circuits you can pigtail each neutral to the device and you should be fine. You can share on the line side. These devices are "blind" on the line side. All would be legal but only one will work!

I have noticed that using split circuits on arc fault breakers will do as you are describing. The breaker is "seeing " an imbalance, which it will on the split circuit, and trips.