Is it possible to design a three-phase delta generator only with two generators?
Consider the three phases of an isolated floating 'three phase' generator, driving a balanced three phase load with no neutral.

Now ground one of the phases.

We still have three phases, with the same voltages between them. The balanced three phase load will still draw the same power, still see the same phase to phase voltages. 

However, we only have two phases which are not at ground potential. So we can drive these two phases with two suitably phased single phase generators. 

I've often done this to drive a 3 phase synchro from my 2 channel sound card, generate two tones with 60 degrees phase shift between them, and use ground as the third phase.