Example:
using (var client = new SshClient(Server1IP, 22, Server1Username, Server1Password)
{
client.Connect(); // Connection Desktop-Machine to Server1
var port1 = new ForwardedPortLocal("localhost", 2222, Server2IP, 22); // Forward the SSH-Port of Server2 via Server1 on the Desktop-Machine
client.AddForwardedPort(port1);
port1.Start();
// Do your stuff, Open SSH/SCP/SFTP
var client2 = new SshClient("localhost", 2222, Server2Username, Server2Password);
client2.Connect();
}