Connecting the spirometers in series gives variable results, depending on the position of the spirometer in the assembly. Individual tests are therefore essential, as results are not interchangeable.
Lefebvre, Q., Vandergoten, T., Derom, E., Marchandise, E., & Liistro, G. (2012). Technical assessment of spirometers connected in series. Respiratory Care, 57(8), 1273-1277. https://doi.org/10.4187/respcare.01464 (Original work published 2012)