You need to enable reusing of the control session. This option is enabled by default since Rebex FTP/SSL release 2015R4.
Older versions of Rebex FTP/SSL client actually do support SSL session reusing (and it's enabled by default), but they only reuse data sessions, not the control session. Fortunately, reusing the control session can be enabled even under the old versions using the
ReuseControlConnectionSession option like this:
var ftp = new Ftp();
ftp.Settings.ReuseControlConnectionSession = true;
Please give this a try and let me know whether it helps.