This is a tough one - there is no automatic ability to do this.
Are there carriage returns between the lines in field A or is the growth due to word wrap? Or is it a combination of both?
If it's based on carriage returns, then it's pretty easy - you just count them and create a formula that adds that many (chr(13) + chr(10)) to the beginning of field B.
Otherwise you have to determine the average number of characters in a row in the display of field A. Then get the length of field A and determine how many CR/LF's to add to the front of field B. However, because of the way proportional fonts work (each character is a different width) this will not guarantee that field B will always be at the last line of field A. There will be times when it is a line above the last line of A and others where it will be a line below.
-Dell