[jeff5may]

In the parallel arrangement, the units can still be balanced against each other to equalize refrigerant flow. They will tend to frost up simultaneously; as the more efficient evap frosts up, it builds up back pressure due to its expansion valve closing.This increases the flow to the clear evap, which will then frost up until it develops back pressure... Process repeats until both are clogged enough to call for defrost. Upon cycle reverse, both fill up with warm gas and the colder one "sucks" more gas until both are free of ice.

Also with series-connected units, there is no real way to balance heat transfer. Due to the single path through both units, superheat control may not be feasible. One unit will always beat the other out of its share of the load. In a water exchanger with counterflow, this effect can be minimized, but in a crossflow air exchanger with equal intake temps, it's nearly impossible.