Connecting the primaries of two 110V transformers in series across 220V
You cannot guarantee that the primary current taken by one transformer will be close to the other transformer. This inevitably means that when primaries are put in series, one might hog more voltage than the other. You can try this on a 110 volt supply and one primary might receive 60 volts whilst the other only receives 50 volts.

The above describes the off-load scenario. It’s worse when loads are connected because if the loads are different values for each transformer, there can be a much bigger difference in received primary voltages and this could be a fire risk.

My advice is this: do not do it.