What is meaning of "tandem" for steel light pole hydraulic press brake?

When we talk about steel pole forming, anyhow we can't avoid tandem hydraulic press brakes. while what is the meaning of "tandem"? Just put 2 or more units press brakes together? what can be the differences between CNC or NC type tandem?

• CNC or Elelctric-hydraulic synchronized hydraulic press brake
  each unit machine is electric-hydraulic synchronized model (WE67K). each press brake machine's controller can control Y1, Y2 by electric and hydraulic system. multi machines will be linked with CNC system. so that each cylinder will be work as one axis, e.g Y1,Y2, Y3, Y4... Synchronization is automatic at any conditions. the performance is the best. 
• NC electric-mechanical synchronized press brake
  each press brake is mechanic (torsion bar) synchronized. synchronization system is not closed loop. so operators shall be careful with performance at beginning or during.