I'd be concerned that you have a large enough supply to run both the furnace and
HW heater at the same time.

Should not be a problem since that is what I had originally. The pipe
running across the ceiling is 1", then there is a 1"X3/4"X3/4" Tee.
One 3/4" drop comes by the furnace (which then Tee's to 1/2" to feed
the furnace). The other 3/4" went on to feed the old HW heater/dryer/
stove.
What I did was add another 3/4" tee by the furnace drop. Plus I am
eliminating the old HW heater.
So I'm not changing the size of pipe or adding on something else. The
volume of gas stays the same.