I think you meant "facing" not flanking?
I did, yeah.
Correct, I misread that earlier. But (as you'll read below) unless you attack your movement hasn't ended and thus the orc won't use its trigger.
I was thinking about the "ending" movement. Technically, if you have two attacks you are ending your move twice, so I don't see any issue with that and how your scenario plays out--so nothing to get into on that IMO.
But without an attack (which is the only way you can split your movement, unless you
dash or something maybe

), you are still moving and it wouldn't have ended yet (so, no triggering event for the orc). 5E
doesn't allow you to split it [movement] up however you like turning your turn.
The thing is, the way the rules for movement in 5e are written, it isn’t a discrete activity with a start and an end. It’s a resource you expend to change your character’s position. It’s actually something I find to be a bit of a flaw in the way the Facing rules are written, since short of a creature running out of movement, there’s no mechanism in the rules to indicate when they’ve “ended their move.” My interpretation is that, when a creature stops moving, you can use a reaction to change facing. If the rogue spends 15 feet of movement to get behind the orc and then stops, the orc has an opportunity to use a reaction change facing. The can either take that reaction, in which case it will turn around and then the rogue can spend 15 more feet of movement to get behind the orc again and attack, or the orc can not take that reaction, in which case the rogue will simply stay where they are and attack. Either way, the rogue can attack the orc with advantage.
With marking though, (assuming the orc has made a melee attack against the rogue since its last turn), the orc can at least make an opportunity attack against the rogue - with advantage - when they try to move behind its back, making this rather silly tactic no longer viable.
Now, you may not agree with my interpretation of the rules, and that’s fine. But that is how I interpret them.
LOL that's a pretty loose interpretation IMO and as I player I would not be cool with that, personally. After all, the attacker is moving into your blind spot so making an attack against it as an OA (especially with advantage via marking) seems a bit far-fetched.
I mean, it seems like you are trying to create a more realistic and tactical situation, but I don't see how (realistically) it makes sense that if someone is moving behind you, you get a chance to attack them. shrug
Makes about as much sense as getting a chance to attack someone when they’re moving
away from you, in my opinion. But regardless, “more realistic” is not my goal. More tactically engaging, yes; more realistic, no. I don’t think realism is a useful goal in game design.
Ok, let's see if I have this right...
- The fighter moves into the rear position (red). The orc can either make an OA (with your interpretation) or change facing. Note: the orc has not attacked yet, so hasn't marked the fighter, so the OA won't be free or with advantage. So...
- The orc changes facing to deny the fighter advantage. Turning to keep the fighter and rogue both on its sides (yellow).
- The rogue then moves into the rear position (red) to attack with advantage as an unseen attacker since the orc cannot turn again.
View attachment 130854
I would say the orc made a bit of a tactical error by using its reaction to turn and face the fighter in fig. 2. By doing so, it has exposed its back to the rogue, which the rogue exploits for advantage in fig. 3. The orc can see that there are two opponents poised to flank it, so it knows one way or another, one of them will be able to get behind it and exploit its blind spot. That means the orc has to decide which opponent it’s going to allow to get at its back. Now, maybe if the fighter has multiple attacks it would be better to expose its back to the rogue, but if not, it’s probably smarter to let the fighter attack its back in order to keep an eye on the rogue. Especially since doing so would free up it’s reaction to make an opportunity attack against the fighter as they pass, which would also mark the fighter until the end of the orc’s next turn, giving the orc more options to punish the fighter’s next move.
Now, how is this ANY different from the set up for normal flanking without worrying about facing:
View attachment 130855
The end positioning you arrived at isn’t any different (though again, I would argue the positioning you reached in the first example was due to a tactical error on the orc’s part), but the former has several more decision points involved. The orc has to consider whether to make an attack of opportunity against the fighter or to save its reaction to turn. It has to consider whether it should allow the fighter to get into its blind spot or expose its blind spot to the rogue. And the fighter and the rogue have to try and anticipate what the orc will do if they want to take the most advantage of their own positioning.
Honestly, it seems more like you are making things convoluted with only the semblance of tactics.
Now, I understand compared to normal flanking rules, only one PC would gain advantage with via the facing rules. I think that part is great because a big issue with flanking for many people is that its too easy for everyone to get advantage on their attacks.
But, what am I missing? I am not concerned about marking, frankly, as I would never use it without some sort of action cost, but that is just my personal preference. So, other than limiting "flanking" advantage to the rear position only, what does all this accomplish?
It gives the players (and me) more factors to consider in positioning our characters. More decision points, more need to try and anticipate your opponent’s moves. As you said, it limits one attacker to gaining advantage rather than both, which is a significant plus. It also makes hiding in combat a more viable option for rogues, which as a big rogue fan I appreciate.