Kamui and Tobi's space time jutsu are two completely different abilities.i think tobi's dimentional jutsu is MS by nature..it is amasterful and advance application of kamui!!!
Kamui:
1. Ranged attack which forms a space time barrier around an object and puts it into another dimension.
2. Kakashi can only use it a few times before he's completely exhausted and the bigger the object he uses it on the more chakara it requires for him to use it.
3. Kakashi does not have the ability to make himself intangible or warp himself/objects back and forth between dimensions.
4. Thus, Kamui is highly specialized for one thing.
Tobi's jutsu:
1. Short range attack which shifts his body or objects between two dimensions.
2. He has been using this technique 24/7 without showing any signs of a drain on his chakara.
3. Tobi has not yet shown the ability of being able to use his space/time jutsu at long range.
4. Thus, Tobi's jutsu is not really highly specialized, it's more along the lines of a general purpose time/space jutsu.
As far as Tobi calling it 'Kamui,' I'm pretty sure it's no different than any other jutsu, where the word 'kamui' is some literal word that describes what the jutsu does. If Kakashi and Tobi could both shoot fire out of their asses, they would both call it flaming ass balls or something like that.
When Tobi shouted 'kamui' at the same time as Kakashi and activated Mangekyo, I'm pretty sure he did so because he intended to use the same technique that Kakashi used in order to teleport something into the next dimension at long range.
Kamui is probably the highly specialized jutsu which is unique to the MS that both Kakashi and Tobi obtained from Obito's Sharingan. However, Tobi's other space/time technique is probably completely independent of Obito's eye. He likely had it before he got Obito's eye, and I'm guessing it's the main technique of Izuna Uchiha.
The reason for this logic is that space/time jutsu are not unique to Mangekyo Sharingan (Tobirama Senju, Minato) and Tobi's use of space time jutsu are far too liberal to be on the scale of MS abilities which pose a risk to the user. If his jutsu was from MS then he'd be blind by now, just as Kakashi is suffering from eye damage after using Kamui his last time.
Lastly, I'm 99% sure that Tobi didn't actually get a chance to use Kamui before he got outsmarted by Kakashi and Naruto using a combo attack.