Check on Instagram an electric supercharger setup.
They did it in parallel.
https://www.instagram.com/p/BwMp6anpdVi/
About the twin charging in series.
A compressor map shows the pressure ratio instead of boost pressure.
So I expect when you put 2 in series you have to multiply the pressure ratio's.
1.4 * 1.4 = 1.96, so 0.96bar boost.
This will nearly double the air mass flow and therefore I expect a drop in pressure based on the same compressor map.