Maybe I'm not an expert, but I am unsure what it is you wish to accomplish. I do not want a block of code, just the bussiness logic. As I understand it you have TableA, which can have records inserted. When a record is inserted in TableA another record should be inserted in TableB.
Now comes the tricky part: do you wish to do an update to TableB if TableA is updated, or the other way around, or something else? You talk of INSTEAD; are you suggesting TableA might be updated BEFORE the value to be updated is actually committed?
Care to explain?