Auto Update a Multivalue Field

I am looking for a way to auto update a multivalue field based on what is in one record. If Record 1 fieldx= 2,3 Then Record 2 fieldx= 1

Record 3 fieldx= 1

And vice versa. I would also like to further edit each record without losing data. So if i want to add another value to record 2 field x then it should read 1,4 and not just 4(the new value)