It is unlikely that your guppies are trying to mate. Male guppies are constantly chasing females in an attempt to mate, but if you only have one male and one female, the female will likely become stressed and may even die from the constant harassment. If you want to keep guppies, it is best to keep them in a group of at least 6, with a ratio of 2 females to 1 male. This will help to spread out the attention of the males and reduce the stress on the females.
There are a few other reasons why your guppies may be chasing each other around the tank. They may be trying to establish a pecking order, or they may simply be playing. If the chasing is not aggressive and the fish do not appear to be stressed, then there is no need to worry. However, if the chasing is persistent and the fish are showing signs of stress, such as hiding or clamped fins, then you may need to take steps to reduce the aggression.