How to pass common inputs to a merged model in Keras
I found the answer to my question. I need to pass the common input shape to the individual models and concatenate the outputs like this:

input_shape=(img_rows,img_cols,3)
commonInput = Input(input_shape)
out1 = model1(commonInput)    
out2 = model2(commonInput)  
mergedOut = Concatenate()([out1,out1])