Secondary voltage with parallel, mismatched transformers
If you used equal value resistors (say 5 ohm in value) to connect the two transformer outputs then the junction of the two resistors would indeed be 9V RMS with respect to the common secondary terminal. Unfortunately in doing so you are using nearly all the available energy that the transformers can supply but if this is for a low power application you can do it. You could also do it with any equal value resistors but don't go lower than 3 ohms or you'll be exceeding the current output on the secondaries and they will start to get quite warm.

Low power applications fine, rated power applications not so fine.