Hi James!
You're right that aldosterone and ADH both have similar roles, but the mechanism they use is quite different! You can think of ADH or anti-diuretic hormone as something that prevents water from being lost at the kidneys. So actually, ADH decreases urine volume. It does this by increasing aquaporin channels in the kidney tubules, so that more water is reabsorbed out of what eventually becomes the urine.
Aldosterone on the other hand acts by reabsorbing sodium in the kidneys, and getting rid of potassium in exchange. Anytime sodium is reabsorbed, water will follow it - also decreasing urine volume.
Hopefully this helps!