Each element in the Tungsten and Cobalt series of heaters must be connected to 220-240v. 

If the element is not glowing brightly, then yo did not connect 220-240v to it, rather you split the 220-240v into 110-120v, and are underpowering (by half) the element. More electricity, more glow.

There is no "Neutral" with 220-240v. Therefore, there are two hots and one ground.

To operate a 4000w or 6000w heater (both have two elements) from one switch, Pigtail N1 and N2 for Hot #1, Pigtail L1 & L2 for Hot #2. No Neutral. Need one dedicated 20 amp circuit.

To operate a 4000w or 6000w heater (both have two elements) from two switches (one per element), you need two 15 amp switches.