Site to site transfers can only be performed between two FTP servers.
FTP and SFTP are two completely different protocols that are not compatible with each other and in order to perform a true site to site transfer the protocol must be identical and must be capable of site to site transfers.
The FTP protocol by design allows site to site transfers.
The SFTP protocol doesn't provide any method of performing site to site transfers.
A true site to site transfer is transferred directly between the two servers without first downloading the data and then uploading it.
A site to site transfer can be simulated by transferring the file through your connection (by downloading the file and then uploading the file) however this feature is not offered by FlashFXP.