Possible to fine-tune just one concept with multiple class dirs?

I want to fine-tune a model using the dreambooth full model training method on pretty much the same thing, only with different class image sets. I’m wondering if I can do this via class_data_dir + class_data_dir2 or if I would have to do it via two different concepts with the same instance data dir. I’d like to avoid the latter as it seems like it would overfit faster.