Can two 61F Floatless Level Controllers and two Electrode Holders be used in the same tank?

Yes, they can as long as the following conditions are met.

The interelectrode voltages are the same.

The minimum separation between the Electrode Holders depends on the type of liquid, but separate the Electrode Holders by at least 1 m.

Use the same phases at the power supply terminals (S0, S1, and S2).