You can charge either targeted unit.
Split Fire allows a single model within a unit to fire at a different target.
That model is still part of the unit, and the unit still counts as having fired in every other way (can't run, can't charge if a non-relentless heavy weapon fired etc). If you want to argue that the split fired model doesn't count as being part of the unit in terms of one thing in the shooting phase (targeting) then you have to also make the case that a split fired heavy weapon marine can stay behind whilst the rest of the unit does an assault without him. Good luck with that one.
The most argument that could be made for picking which target you will be able to charge before resolving the actual shooting (taking into account that at no point is such a requirement stated, or even implied, in the shooting or split fire rules) is the term 'the unit that was targeted' in the charge rules. However, that word choice is negated later in the multiple assaults section, so not the strongest bargaining point...
For more amusing fun, consider the implications when Psykers with Witchfire are also present in the unit- each witchfire power can target a separate enemy unit.
Theoretically a unit could have dozens of allowed charge targets!
Personally I think the entire 'only charge who you shot at' rule is bunkum.
Imagine a unit of marines is faced with a unit of Orks, and a unit of Gretchin in the way blocking their charge path. They open fire on the Gretchin to clear a path through to the Orks, and successfully do so by wiping out the Gretchin.
Why the hell would they then stand around going "Well that small arms fire did well, cleared us a direct path through to those Orks. Better stand around and compose ourselves for a few minutes lads, it's not like we are in the middle of a war and already heading in that direction with our chainswords drawn or anything." ?!?
Why?
One was targeted by a model in the unit, one was targeted by the unit. A unit can only charge what it targeted.
The simple fact is that the unit actually has a target declared by the time it is ready to charge. And the unit is never stated as targeting what the model targeted. And because it is just one model and not the unit targetting, it is not a valid primary target for a charge.
Really? From Declare Charge:
"(A) unit that fired in the Shooting phase can only charge the unit that it targeted during that turn’s Shooting phase."
Yeah, it's not in the Shooting Phase rules or Split Fire Rules, because the Shooting general rules are not concerned with what happens afterward, and neither does Split Fire (though, it should address it, one way or the other, but that's GW for you). It's not even until we get past the Shooting Phase rules that we get to the Weapons that the rules concern themselves with the consequences afterward (i.e. Rapid Fire, Heavy, etc).
Declaring a charge does not care what you Witchfired, though, only what the unit shot at, as I referenced above.
Good, because that is not the rule. If it was a rule, this wouldn't be much a discussion. It's what you targeted.
I think it's pretty stupid that a Lascannon Heavy Weapon Team can't concentrate fire on Tanks while the rest of his squad is using their [s]Markerlights[/s] Lasguns on Infantry. But those are the rules.
Are you trolling, or just honestly this dense? The model using split fire is part of the unit, not a separate entity. The model chooses a separate target from the unit to fire at, then the unit fires at a different target. Both times the shots are coming from the same unit, and both times require the unit to target something. What about this is hard to understand?
Are you trolling, or just honestly this dense?
While the model is part of the unit, a model's actions do not always represent the unit when it does it. You even repeat the situation and then deny it. The Shooting Attacks are coming FROM the unit, but only one is targeted by the unit, the other is targeted by a single model in the unit. That is literally what the rule states. One model targets X unit, the rest of the unit target Y unit. Y unit is the target of the unit, not X unit.
Remember, there is a difference between targeting and shooting, and the unit, as a whole, only targets one unit as normal.
This is a perfect example of basically every rules question that comes up on this site. Some clown has an extreme view about a rule that is based off ignoring every precedent in the rules and willfully twisting how the game works, then defends said bad ruling adamantly against everyone. Even when (especially when) they are shown the actual text from the book, they double down on their position. Then someone (we all know who) posts it to the main blog as if it's a real problem that players are facing at tournaments and gaming tables. Rinse and repeat.