So if we assume normal O5e rules, only the attacker would get an OA here.
Attacker Presses the Attack
Defender falls back. Assuming this was at the edge of the attacker's reach, this would move you outside of that reach, triggering an OA.
Attacker than moves in to complete the attack. As this is a movement within reach, it should not provoke an OA (this is a specific O5e change to OAs, different than 4e and 3e, which worked when you moved through reach). Even if it did, the defender has already used their reaction to fall back, so they wouldn't get an OA regardless.
I wasn't meaning that they would trigger OAs to each other as I'm seeing their movement pretty much lockstep.
I was meaning OAs wrt other combatants in melee. Imagine there's a 2 vs 2, both the pressing character and the falling back one would trigger AO from the enemy combatant not in front of themselves.
Considering the OA AND the penalties the defender gets, I think its going to be a very rare for a defender to want to fall back. I think it would have to be used with some kind of team combat tactic (such as another ally getting an OA on the attacker should they close in on the defender), or if the attacker was immobilized, etc. But 1 on 1 especially, I would struggle to think of many scenarios where I would want to fall back.
If the falling back character would trigger an OA from the pressing one, I agree with you: it would still be attacked (without advantage), and also receive an OA from the pressing enemy.
If the falling back character wouldn't trigger the OA from the pressing enemy than I see the falling back option becoming more useful.
But very likely there are many tactical aspects we're not considering right now, and that could change our conclusions quite a bit.
For example: fall back specifically says that the pressing character does not have advantage. If you also have some combat maneuver (stance) that allows you to impose disadvantage on attacks, and also maybe make counterattacks or gain some other form of edge when the enemy misses you, then that could become an interesting semi-passive fighting style.