BioBERT base and LegalBERT use the same architecture so using a technique like Git-rebasin would improve performance over using just one or the other model, however if you want to merge the models and get the best of both models, you should retrain on a merged dataset or use model ensembles instead (aka, load and run both models and intelligently pick which model to listen to for which type of data)

You can not (easily) merge BioBERT large since that checkpoint uses a custom vocabulary, but BioBERT base looks perfectly fine.