Advantage on flanking would suck as advantage is a horrible mechanic that doesn't work with itself. Barbarians and monks giving themselves advantage every round would get nothing from it.There are no flanking rules as of yet. If/when they appear it will be in the DMG. I personally would not hand out advantage for something as easy to get as flanking in 5e, advantage/disadvantage is something I give out just a couple times an encounter not every round. The current rules, one of the two flankers could use the help action to give the other one advantage. I might give advantage if the target was out numbered by 4 to 1 by opponents equal to his size, or so like a gang up rule.
If you are flanking an enemy, you can use a bonus action to perform the help action.
Thinking on this some more, I came up with the following.
If you are flanking an enemy, you can use a bonus action to perform the help action.
To me this works well because it keeps it as an advantage/disadvantage thing not a fiddly number bonus like a +1 or +2, it uses up a bonus action so it takes some effort on the flankers part, and it only applies to the allies first attack against it not all of them.
For now that will be my house rule, if one of my players asks.