There's no perfect solution for mixed planting, but there is a sensible approach:
Assign the sensor to the plant with the highest water and nutrient requirements, or the one that's most sensitive to under- or overwatering. Think of it as the "reference plant" for the measurement.
If the plants have very similar needs (e.g. different herbs, or chilies and peppers), just pick one - the recommendations will work well for both.
One important note: if you're using multiple sensors in the same pot, keep a minimum distance of 50 cm between them to prevent interference. For smaller pots with mixed planting, a single sensor is usually enough.