Product review (4 stars): Work well and can be used in multiples.
I bought two so I'd have two readout units, but use only one of the small sensors. The two units have no trouble picking up the signal from the one sensor. This way I can see the outdoor temps on both floors of the house. I have no idea what sort of cross interference would occur if you tried to use two sensors and two readers; doubt they have different frequencies to sort out which signal to read.