My house was built in the early 60s, I have 4 circuits that are created by using 2 3 wire cables, each hot leg goes to a 120v breaker side by side in the panel. The neutral goes to the neutral buss. I don't see any way the breakers could have ever been tied together. The cables split up somewhere but with all the insulation in the attic I cannot see where the splits are. I'm replacing my entrance panel and have to install afci and gfci breakers so at the breaker I will end up short one neutral wire for each pair of circuits. Can I use a short piece of wire from the neutral buss or split the one neutral or will I have to re wire the circuits so each has its own neutral. I have the electric company comming Wed to shut of the power, I just noticed this wireing today while labeling the wires.
I think you are referring to a shared neutral/multi-wire branch circuit. These are commonly found in kitchens. They were and still are to code in many areas, but typically I believe their use is limited to kitchens.
You should run a new cable back to the breaker box. If you were to run an independent cable then it is possible for it to break independently of the rest of the cable, which would leave you with an open neutral which can cause all sorts of problems. Unless your cable branches from another box, you cannot just run a neutral from one box while the hot comes from another circuit. You have to run a new direct wire anyways, so why not just pull a new cable - it's just as much work as pulling a single wire.
If you do opt to keep the multi-wire branch, the breaker should be tied together so that there is not the possibility of one half being off and the other on (a possibly dangerous scenario if someone assumed the power was cut because one of the circuits did not test live). Likewise, if it trips because of a short, there is a good chance the other hot is close by and you want the second breaker to trip too.