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.
I'll have to think about that one a bit. I've been going with advantage for one of the allies, provided the first ally hits with their attack. I've also considered allowing the 'helped' ally to move out of reach without provoking an opportunity attack.
Originally I was just going with the Help rule, but that meant that one of the allies couldn't attack. That didn't really make sense to me, because if you weren't taking an attack then you wouldn't be drawing the enemy away from your ally very much. Yes, I know that descriptively you could still be attacking, and just not hitting.
Another alternative I thought about was to use the Help action, and allow a bonus attack instead. This would limit you to a single attack, instead of whatever multiple attacks you'd normally get.
I don't particularly mind granting advantage or disadvantage. Combat is often all about trying to put yourself into an advantageous position, and goes back to the simple rule of earlier editions to not worry about all of the modifiers and just go with a +2 or -2 based on who has the advantage. Of course, the monsters are usually trying to get the advantage as well.
Since Combat and Tactics, though, flanking has been a staple tactic (rightfully so), so I do want to go with something.