There is no rule per se, but I was advised that if a figure from the 2nd rank attacks a figure from reach, & is not within 5 feet, then do NOT count them as Engaged.
However, in my game this has allowed for spearman in the second rank to back up on their turn, and then charge over and over again (for they never become Engaged), if they have sufficient room.
Firstly, when charging, ONLY 20 feet of the distance must be in a relatively straight line, correct? The rule says you have to have line of start from the get go. I add to this, in that you must maintain line of sight throughout the charge. Thus, I know that part.
Normally, this back up and charge maneuver couldn't be done all that effectively, given the combatants would most likely become Engaged.
I don't know if this is a feature or a bug. I want to control for them NOT doing it, without having to modify every encounter. I also do NOT want to consider the combatants Engaged - I don't think - given that I was advised by a Autarch Not to, but I'm unable to come up with a solution other than always having to be cognizant of it and designing encounters accordingly. If that's what I have to do, that's what I'll do, but I'm looking for a 3rd option (or more).
All that said, I'm not positive it's a problem. Especially, given monsters with reach can do the same. But it's just kind of funky imho.
If you spend a turn backing up, then a turn charging for double damage, you're not getting much advantage. You get double damage one turn, no damage the next. You get the charge bonus, so you'll hit slightly more often. But you can do all that just by conducting a fighting withdrawal (the front rank will prevent the enemy from following, even if you are engaged).
Personally, I wouldn't allow a second rank to benefit from a charge unless the front rank is also charging (otherwise your momentum is going to carry you into your allies in front of you).
My reading is that charges need to be "in a relatively straight line" and "a minimum of 20' ". Not "a minimum of 20' in a relatively straight line, plus any distance running in a serpentine fashion". If you're suggesting you can back up and then charge all in the one move, I would say absolutely not.
Isn't the problem moot for most characters? First, they have to move 20' back, then 20' forward. The number of characters with a Combat Movement of 40' under the standard rules seems limited - even with the Running proficiency, keeping a Combat Movement of 40' means not carrying much and taking limits on AC. Standard adventurer kit is usually worth about 2 Stone in encumbrance, before taking weapons or armour into account, so almost nobody who's wearing armour is under 6 Stone and nobody in heavy armour is under 8 Stone., Which means, unless they're dropping every piecce of noncombat equipment (and where are they dropping it? and who's first to pick it up?), few characters can pull this off.
Also, the few that can pull this trick off require: a. armour capped at Chain Mail plus a Spear or Polearm, b. 20' space to back up for the charge, c. someone standing in front of them, stopping the enemy from Engaging them, d. a safe spot to dump anything that might slow them down in combat.
That sounds like an inherent limitation on the tactic to me, but that's just my opinion