RAW, DM decides.
On page 175 of the 5e PHB, adjacent to the section on passive skills, there is a section called 'Working Together'. This states that you can grant advantage to someone by aiding them, and calls out the 'Help' action as the in-combat version of this. However, it adds a caveat that this can only be the case if someone helping you is.. helpful, giving the example of threading a needle as something where you need to be able to do it alone (although someone could thread the needle for you, assuming normal needle-threading conditions additional pairs of hands do not make the job easier).
I would say that someone else looking for enemies, traps, or other points of interest does not make it appreciably easier to spot enemies, traps, or other points of interest. It makes it easier if they already know where one is, and you are having trouble spotting it still - they can point it out, for example. But two people looking for enemies is just two people looking for enemies - they are not meaningfully aiding each other, they are both attempting to complete the same task (such as two people trying to thread a needle each). However some people will almost certainly feel differently ('I look left, you look right') - whichever way the DM decides, it is pretty clear from the text, is how it goes - it's explicitly a judgement call.
Mathematically, it shouldn't.
Passive perceptions are a bit of an awkward kludge as it is, removing the randomness from the game for a very important thing (stealth mechanics). Bob the Eagle Eyed Ranger is ALWAYS going to spot enemies and Bib the Halfling Thief is NEVER going to spot enemies, with their +6 and +1 scores respectively. While you can and should have people just notice things if they've got time to look around ('you didn't SAY you were looking for treasure, so you left that staff of the archmagi on the ground' is garbage DMing) always having Bob notice things and get to interact with them first etc and never Bib is lame. For important things, rolling your own dice to determine who sees what is usually far superior, as then Bob will be rewarded for his eagle eyes (usually being the one to spot things), but occasionally Bib will still get a turn with a lucky roll. Which is one of the many reasons dice are in the game, so things have that element of randomness.
Adding advantage to perception tests (or passive perception) means that whoever has the lower modifier shouldn't ever be doing anything but assisting. It might mean the party spots things better, but it will make sure that Bib never, ever, gets a turn to see something and it is always and only Bob who does so. It doubles down on the 'biggest modifier wins' dynamic and promotes the 'everyone goes everywhere together and only whoever has the highest modifier rolls for anything' style of play, which is one of the most boring.
It will also nuke the expected stealth values of the game completely out of the water (unless stealthing creatures are somehow receiving their own assistance) making ambushes, traps, and other key parts of D&D a thing of the past.
Overall, it should not be allowed, for many reasons. Making the game procedural rather than random, spotlight hogging, destroying the basic math assumptions of the game, removing parts of D&D by making them extremely mathematically unlikely, etc.
