Formation of bubbles from where?

The series pump setup increases pressure due to 2 stages. If one motor was strong enough, both pumping elements could be driven by the same motor giving the same effect. Pressure is only built when the outlet is restricted and the resulting increase in pressure is shared between the two pumps. ( Both pumps have the same part number. )

Gravity >> pump 1 + 50 PSI >> 50 PSI >> pump 2 + 50 PSI = 100 PSI