I thought that at first too, but then a passive check is an action, just a repeated one, over and over, and the passive check represents the average of rolling 10. So, if you have 2 PCs helping each other keep a lookout over a few hours march.... why shouldnt one get adv, which translates into a +5 bonus for a passive check. I think it works. But I can also appreciate ruling the other way.
As far as "allowing" passive checks, they are assumed default in the rules, it is the taking a 10 of this edition, it is built into the stealth and exploration sub systems. Not allowing passive checks in your game is like not allowing backgrounds, sure as a DM you can do it, but we are not talking about something with an "optional" tag here.
Multiple people can be noticing threats, there is talk about not contributing to the perception check so I assume the default is if you have two people doing this you do use the help rules and take the highest passive perception and add 5 to it, if the party moves at a fast pace they take a -5 penalty so this seems to counteract that but only if the party needs to move at 4 miles per hour instead of 3 miles per hour.
Both of these sections indicate that the characters are all working at the same task (noticing threats), unless they choose to focus on some other activity. And even if so, the characters use their separate Passive Perception scores, instead of pooling them up into a single score with +5 for advantage.
There is no point of just using individual passive perception checks in a group activity like that, the highest passive will always be the same and the other members in the group on look out will not contribute, so having a second person help and give the +5 bonus is the only thing that makes sense, but we will see for sure when the DMG comes out and the topic of exploration is gone into more detail.