"If you're not with me then you're my enemy"
"Only a sith deals absolutes, I will do what I must"
What happened to the Rule of Two? Let's say Obi-Wan accepted, there would be 3 Sith then.
Given that one of the main points of the "Rule of Two" was to allow the Sith to remain concealed (lest the Jedi find and kill them), I'd say that the need for that particular rule had simply become irrelevant.
Secondly, it's by no means clear that Anakin views himself as a Sith at this point, nor would he necessarily have been indoctrinated into the deeper mysteries of Sith history. He may simply not have known of the rule.
The Rule of Two
The Sith "Rule of Two" was originally put in place by Darth Bane to prevent in-fighting amongst the Sith. Even with a common enemy in the Jedi, it became apparent to Bane that the Sith themselves, due to their power-hungry & competitive nature, were the true enemy.
To combat this, he conceived of a system in which only two Sith could formally exist at any given time - a Master and an Apprentice. When the Master died, or the Apprentice grew strong enough to overcome the Master, the Apprentice became the Master and takes a new apprentice. Under this system, there is never more than one heir to the title. However, the rule has never seemed to prevent either Sith from taking on additional apprentices (usually in secret) or from having other Dark Jedi allies.
Had Obi-Wan supported Anakin's actions, he would have likely fallen into the latter category - a Dark Jedi, but not a Sith. Under those conditions, he would have operated as an ally of Anakin, who was formally the Sith Apprentice under the Rule of Two. Anakin's statement "If you're not with me" can be taken as "If you don't support me" - i.e. not as an actual Sith Lord, but as an ally & friend.
Anakin didn't want to kill Obi-Wan. He fully expected that Palpatine would take care of it, and only because it was necessary.
Anakin didn't necessarily want the death of the Jedi, but he was convinced that the Jedi were going to take control of the senate by ousting Palpatine (a situation Palpatine manipulated himself so he could justify the killing of the Jedi).
As far as Anakin was concerned, it wasn't a choice between becoming a sith or staying a jedi - it was a choice between supporting Palpatine as the ruler, or trying to oust Palpatine in some sort of coup.
Yet he did not want to kill Obi-Wan. So he was hoping to be able to convince Obi-Wan of his position on the matter and have Obi-Wan change his mind on getting rid of Palpatine.
Obi-Wan knew at this time that Palpatine was a sith, and that he had successfully convinced Anakin to become his apprentice. Anakin, a teenager still, may not have understood what all this meant, though he did follow through with his commitment to lead the assault at the jedi temple.
Further, Obi-Wan was having difficulty himself believing that Anakin had truly become a sith or accepted the dark side. Despite the evidence, including the video footage of Anakin slaughtering younglings, he was still internally struggling with the idea that his apprentice had gone that far down the path of the dark side without Obi-Wan noticing.
So Obi-Wan's statement, "Only a Sith deals absolutes" has two purposes: 1) It is an open accusation to Anakin that he is a sith. Before this time, no one has told him that this is what he has become, though he understands he's in the game to learn the power to save Padme, it may not be until this moment when he truly hears from someone he trusts that he has become what he used to consider the enemy. 2) It is Obi-Wan vocalizing a truth he has refused to fully accept until now. Anakin has given him an ultimatum. Prior to this their fighting was "aggressive negotiations" but with this statement Obi-Wan stops attempting to understand and be understood, and steps fully into his role as executor of a Sith.
The "rule of two", even if Anakin was aware of it - and as a high ranking Jedi he likely was - simply wasn't a consideration. He wasn't attempting to create a sith, he just wanted his friend to accept Anakin's decision and join him in ending the war so he wouldn't have to kill Obi-Wan.
First, there's no reason to assume Anakin even knows about the Rule of Two at this point, since he has only just recently become a Sith apprentice. He may not yet know about the fact that Sith only work in twos.
Second, this is exactly what the Sith apprentice almost always does - attempt to recruit another apprentice to help overthrow the current master.
In fact, it's exactly what he plans to do with Luke Skywalker in the future, promising famously to 'rule the galaxy as father and son'.
So there's really no conflict with the Rule of Two here - either Anakin doesn't know about it, or he knows about it, but is attempting to beat it by recruiting an apprentice. Admittedly Obi-Wan would be an older and more jedi-trained apprentice than his son, but since he sees his own actions as being completely reasonable at this point, he'd assume Obi-Wan would at least consider the possibility - never mind that what he's doing is completely evil. His sense of justice has been clouded by the power of the Dark Side.
If so, Anakin and Obi Wan would kill Palpatine and Anakin would take Obi Wan as his Sith apprentice. Anakin would break the rule of two and train Luke and Leia, creating the rule of four. Anakin would become the Emperor, Obi Wan would become a Supreme Commander, and Padme would become the Empress once Palpatine is killed which would bring balance to the Force. Anakin, Obi Wan, Luke, and Leia would all wield purple lightsabers since they are not Jedi Knights but not true Sith Lords either. Anakin would be the true chosen one, from a certain point of view.