Do medal effects stack?

I believe you you guys are wrong. Many medals do stack. The 2 life medals will give two extra heart containers. Also, the cursed medal does not seem to nullify static effects. With the cursed medal in your bag you still get an extra heart container from a life medal, you still get extra ammo from ammo bags, and the bug medal still works with the cursed medal as well. Also other items like ammo bags stack if you have more than one.

All this seems to point to medals and other items stacking. Even with the cursed medal.

The Cursed Medal doesn't nullify anything, just won't let you OPEN your pouch. All items in your pouch are still in effect (you'll still get a higher Bomb and Arrow count, you'll get your extra hearts from the Life and Heart Medals, and the Potion Medal works too).