In a short circuit test, the resistance seen is the primary resistance plus the turns ratio squared times the secondary resistance.
In your example, the 400 ohms would only be split equally if the turn ratio were 1:1 and the primary and secondary were identical. Transformers often have a many more turns on one winding than the other, and the winding with more turns will use thinner wire. However you can't tell how much of the resistance comes from the each winding easily.
In mains isolation transformers with dual secondary windings, the two secondaries will often have different resistances but the same number of turns, as one is wound around the outside of the other, so is bigger diameter.