There is a ton if information available on the topic. Two phase was common
in the old days.
Powerhouses generate 3 phase power, 1 phase goes down your street to a
transformer that's center tapped, giving you 120 and 240v. The two 120
circuits are 180 degree apart but the circuit isn't out of phase with
itself. Real 2 phase is 90 degree out of phase and usually used 4 wires.
Boring stuff, to me.

Then why do you write about it?

Yes, the generally accepted term for the power delivered to your house
from a center tapped transformer is split-phase. Yes, what is commonly
called two-phase power is 90 degrees between phases rather than 180, but
my point is that this is semantics to differentiate the two and a split
phase system really has two phases just the same as a "two phase"
system. The phases simply have a different separation angle.


Matt