Advantage/disadvantage is a single mechanic, so I think it should have one master tag. When advantage was made our tag name character limit was too small for advantage-and-disadvantage (26, 1 too many).
Now that we have more characters to use, we should just rename it advantage-and-disadvantage. That would be findable by typing disad
..., so we might not need a disadvantage tag, but because the new tag would be long, it would probably help to have it as a synonym for those who don't notice “…-and-disadvantage” trailing off the end. That way we'd have:
advantage → advantage-and-disadvantage
disadvantage ↗︎
(Also, if we find that we need to use either of the shorter ones for another game that has completely different things called “advantages” and “disadvantages”, like GURPS does for its chargen pieces, we have the option of breaking the synonyms at that later date.)