This falls under the "This also means you determine how an activity works if the character’s actions differ from those on the list." portion of the Exploration Activities rules.
A GM could choose to have two characters doing the same thing (example: searching) roll independently thus providing a dramatically increased chance of the party succeeding at that task... or the GM could use the Aid rules which keep the odds of success closer to what they would be if only one character were rolling, but provides potential for higher DCs to be reached.
I lean toward letting Aid work when characters are working together on something like that because I like not feeling like a roll that should have 50-60% chance of success has been given a 75-85% chance instead just because someone said "me too."
Downtime, on the other hand, has a specific rule about Cooperation (p. 500) that says simple tasks count everyone as aiding one character and complex tasks assume individual efforts all working towards a shared goal.
