Wait until you can do both.
Do it right, or don't do it at all.
Definitely don't take the stockers off unless you have a specific need/desire for some specific behavior that that would cause.
What behavior are you wanting to alter in the first place? Too much oversteer, too much understeer, or too much body roll in general?
If you don't replace them both at the same time, you run the risk of inducing too much over or understeer depending on which you do. Just remember, stiffer rear will reduce understeer (too much and the rear will be very loose), stiffer front will reduce oversteer (too much will cause very strong understeer).
Edit:
If the answer is "too much body roll" and you are otherwise happy with the handling balance of the car, you definitely want to make sure you replace BOTH anti-roll bars at the same time with a nicely matched set. That way you will keep your balance.