Converting a dataframe to json and writing it to a File & appending a new Dataframe to that file

I need to write a Dataframe, converted to JSON format, to a file . This can be done quite easily with a one liner df.to_json() . Problem arises when I write it to a file because I need to append to the same file other dataframe as well. How it can be done efficiently

I am writing to the file using dataframe method df.to_json(orient='records',path_or_buf='filepath')

Now if I have another dataframe to be added to this file only then how it can be achieved (without loading it to memory)