Advantages/disadvantages as above.
But if you use 1+1 remember to use the recommended amount of developer per roll or compensate timewise accordingly. Otherwise you might end up with varying degrees of underdeveloped negatives:
(from Kodak's data sheet J-78, Oct 2002)
"If you use D-76 Developer diluted 1:1, dilute it just before you use it, and discard it after processing the batch of film.
Before using the diluted developer, make certain that there
are no air bubbles in the solution. If air is coming out of the
solution and forming bubbles, let the solution stand until the
bubbles dissipate. Don’t reuse or replenish the diluted
solution. You can develop one 135-3 roll (80 square inches)
in 473 mL (16 ounces) or two rolls together in 946 mL
(one quart) of diluted developer.
If you process one
135-36 roll in a 237 mL (8-ounce) tank or two 135-36 rolls
in a 473 mL (16-ounce) tank, increase the development time
by 10 percent (see the following tables)."
http://www.kodak.com/global/en/professional/support/techPubs/j78/j78.pdf
As usual, best will be to experiment a bit, see what works for you.
.