Guild Wars 2

For example:
Peach Pie grants passive healing.
If I have a Rune of Dolyak 6/6 enchanted on all my armor parts (which also grants passive healing), do I get twice the amount of regeneration?

Yes. Both are too low to matter, though.

Depends on your build on whether or not regen is useful. Mango pies along with the right trait choices will give you a good 400-500 health a second regen. As your heal skill recharges every 20 seconds just compare the numbers.

Without the food and regen traits you get 4.5k healing every 20 seconds. With regen traits and mango pies you get 14.5k health every 20 seconds. The difference is very substantial.