Itachi and Hidan's chances of winning boil down to the following:
-Does Itachi know about Hidan's ritual? Since intel is Manga, you'd think Itachi would try to learn more about his Akatsuki comrades, but because nothing indicates this we have to assume Itachi knows nothing about it. But because of how Hidan fights, he'll keep nagging Itachi to let him try to solo to do his ritual. So let's assume Hidan lets Itachi in on his technique.
-Will Kisame restrict water pool for Kakuzu's safe being? I think he will to a degree, but once Hidan and Itachi get the upperhand then he'll say **** Kakuzu and try to take this. SInce Kisame is the type of guy that will sacrifice team mates for the sake of the objective (the objective is to defeat the "traitorous Itachi and Hidan".
-Does Itachi know about Kisame's GSB and water pool? In part 1 Itachi firmly tells Kisame to hold back against Asuma because his techniques attract too much attention. So Itachi might not specifically be aware of water pool and GSB. But he knows Kisame has the chakra load and techniques to change the environment.
^ Now that these 3 things have been debunked let's see who takes this:
-Because Itachi is aware of his former partner's destructive capabilities. his prime goal is to take Kisame out first. I'm under the assumption he'll tell Hidan since he knows Kakuzu better to hold him off.
Now the question here is: How long could Hidan hold his own against Kakuzu? For starters we all know nothing in Kakuzu's arsenal will harm Hidan except a decapitated head. So this all boils down to taijutsu. While Kakashi was able to hold Hidan off from getting Choji's blood, Hidan was capable of kicking Kakashi. So Hidan's taijutsu is no joke, he may be the weakest Akatsuki, but NB definitely underrates him. Anyway, Kakashi was also capable of going head to head in Taijutsu against Kakuzu. This is a fact, and it wasn't until the fire and wind hearts combined that Kakashi was losing. So it's safe to assume Hidan can dodge Kakuzu's ninjutsu excluding fire+wind. But because of who Hidan is he'll probably just tank the hits, it's not like the attacks will harm him either.
Now while Hidan and Kakuzu are fighting. Itachi is aware of Hidan's ritual, he'll try to quickly take Kisame out by getting a blood sample off of him. Itachi can easily do this. Kisame is aware of Amaterasu and Tsukuyomi, so Kisame knows not to look at his eyes. This is why Sharingan genjutsu will be more than enough to buy time to get Kisame's blood.
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it's safe to assume Itachi can do the same, BUT!!!! Kisame was capable of breaking Aoba Yamanaka's genjutsu(?) by biting his tongue, but he didn't do it instantly. Allowing Aoba to see some of Tobi and Kisame's first meeting.
So this is how the fight is going: Hidan is getting his ass spanked by Kakuzu, but is still obviously standing. Itachi has stabbed Kisame with a kunai (the pain breaks the genjutsu), Itachi then backs up after Kisame tries to swing Samehahda. Kisame laughs thinking Itachi was trying to kill him. Itachi then calls Hidan back over, has him lick the Kunai. Kakuzu is now shitting himself and combines the fire and wind hearts to try and stop them. *Itachi pulls out susano'o(if he has to)*. Kakuzu now warns Kisame of Hidan's ritual. Kisame then tries to shoot out water pool. But Hidan has now finished writing the symbol and continuously stabs himself while Itachi's susano'o prevents the water from washing away the symbol. Hidan has stabbed Kisame's heart killing him (I highly doubt samehada can heal that, bullshit hax if he could, if need be Hidan can stab his arm to make him drop samehada).
So now we have an Itachi who is in pain (I doubt he's coughing up blood because he used 2 ms techs against Hebi before susano'o) after holding susano'o up for some time, and a blood covered psychotic Hidan who's over joyed for performing the ritual.
Kakuzu will now realize he has to keep his distance, so now he'll shoot a fire+wind attack to take out a weakened Itachi. Itachi will prepare to pull Susano'o out again BUT WAIT! Hidan tanks the hit for him as a mean of thanks for letting him do the ritual. Kakuzu will then begin going in to long range mode to avoid Susano'o and Hidan's scythe. As Kakuzu and his hearts come together, Itachi lights Kakuzu up killing him and his hearts.
Let's say it's not enough to kill him. While Kakuzu is screaming in pain Hidan comes in and gets a blood sample, constantly stabbing Kakuzu as his hearts try to re-enter his body to keep Kakuzu alive, Hidan eventually kills them all.
Itachi and Hidan high difficulty.
NOTE: Considering Itachi's intelligence it'd be no surprise if he was to pull this off.