Yes – two probes of the same type can be used with the Hach HQ40D.
The Hach HQ40D is a multi meter with two channels – this means it can accept two probes and measure two parameters from pH, conductivity and dissolved oxygen simultaneously.
If you want to compare two samples you can attach two of the same type of probe, e.g. two pH probes. The split screen will then show the results from both probes on screen simultaneously, as shown in the example below with one DO (dissolved oxygen) probe and one pH probe.

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