You haven't said what size your burners are, what size batches you are doing, what your supply pressure is, etc. But I can do some handwaving. A short run of 1/2" on the the brewstand is probably fine for 10-gal batches IME. To the extent that you have a common supply line for both burners you may be happier with 3/4" for the common portion and 1/2" for the runs to the individual burner.