well you will have to use filters to update these correctly.
suppose you have
field1 contains data from connection 1(code, name)
field2 contains data from connection 2(code, title)
you will have to write rules on both these fields to update the other field
1) on field1
field2 = connection2.title where connection2.code = (select connection1.code where connection1.name = field1.value)
2) on field2
field1 = connection1.title where connection1.code = (select connection2.code where connection2.name = field2.value)
NOTE: this is only concept you will have to translate it into actual syntax