It may be a pair. You can quickly tell a male platy from a female by looking at the anal fin. I don't have pictures of platy for sexing but this is a close relative that you can tell the same way. This is a male with the anal fin changed into a long thin member that is used to impregnate a female.
This is a female of the same species with the typical triangular anal fin of a female.
I hope this helps you be able to tell what sex platy that you are dealing with.