When you say you want to join them together, do you mean you will configure them in failover? or just use only one ASA and join/merge the configuration from both ASA? If you want to merge the configuration from both ASA into one ASA, copy tftp run command is not a good option as I believe it would overwrite the existing configuration on the ASA with one from TFTP.
But to answer your question, yes you can copy a configuration file from tftp to the running config using that command just like on a switch or a router.