In a true N phase system the voltages in each isolated phase are at 360/N degrees electrically. In the NA system there are 3 such phases, each 220 V and each at 360/3 = 120 degrees to the other. It is customary to split each of these phases into 2 x 110V portions. Each of these is constrained to be at the same electrical rotation by virtue 9and constraint) of being on the same core of a 3 phase transformer and are unable to be isolated electrically without the provision of an additional isolating transformer. The two split phases do not meet any of the usual constraints usually implied by the term "separate phases"

The two split phases do not meet any of the usual constraints usually implied by the term "separate phases"

In a true N phase system the voltages in each isolated phase are at 360/N degrees electrically. In the North American system there are 3 such phases, each 220 V and each at 360/3 = 120 degrees to the other. 

It is customary to split each of these phases into 2 x 110V non-isolated portions. Each of these is constrained to be at the same electrical rotation by virtue and constraint) of being on the same core of a 3 phase transformer and are unable to be isolated electrically without the provision of an additional isolating transformer.

It would be possible to physically split the winding at the centre point and to provide two voltages, each still in 0 degrees or 180 degrees isolation to each other - but this is not done. Doing this would require that each of the 100V windings be connected to Neutral at one end, and they would then be unable to be connected together to allow the full 220V phase to be used in eg heating applications.