Is this kind of transfer learning is possible in Tensorflow

Hello I’m doing a CNN project for classification of Corn disease images(4 classes) It uses VGG16 as its base model. I have created and saved the model. Now is it possible to use that model as a base for another transfer learning task to classify cotton leaf disease images( 4 classes) with retaining the knowledge gained from corn disease images along with cotton leaf disease images? If so how should I modify the corn disease model. Should I need to make the output layer neurons as 8( 4 for cotton, 4 for corn disease) ?