No, you can train a model that can detect new objects from a pre-trained model. But there is not a way to combine them directly. To create cat+dog.tlt, it is needed to prepare cat and dog dataset. The cat.tlt or dog.tlt can be a good pretrained model during the training.
There is no update from you for a period, assuming this is not an issue anymore.
Hence we are closing this topic. If need further support, please open a new one.
Thanks