To the above I would add: Have a dated paper trail. MOST elders and C.O.s dislike - yet respect- a paper trail.
If a person has already been disfellowshipped for at least 6 months...usually 12 months....A nice simple handwritten (dated) sorrowful letter sent to both congregations asking "what they want you to do to get back in". (Perhaps mention that when business takes you out of town-you attend meetings elsewhere, you are "sorry", and you made amends to any offended party/victim-if any). Mention some "works befitting repentance"...I.threw away the computer, prayer, study, meeting attendance somewhere, yadda yadda...etc.
It will take a few weeks for the two halls to decide what to do and give you an answer. Then show up here and there for a few weeks. 6 mos later write a 2cd short letter saying you haven't heard back, are discouraged, depressed..then attend meetings super regular for a month or two. Then 3rd nice short letter. I have seen a half dozen people get back in after attending only a couple of months of meetings this way. I have also seen Disfellowshipped who have no dated paper trail... wait years.
If both halls REALLY dislike you..give up, get a life and move on. For those needing family it serves the same purpose.."I tried to go back"..."the Elders wouldn't let me back" "I sent 3 letters and went to meetings"....you are now off the hook, your family will now be mad at the Elders, and you only had to listen to the meetings for a couple of months..!
Really no hard and fast rules. A lot of Elders seek revenge or like to see people grovel. ......What a cult.