On a motorbike, it is unusual to have two tyres with the same amount of wear at the same time, as the rear tyre usually wears out more quickly than the front tyre.
Tyre manufacturers invest large amounts in the design of their tyres and ensure that, for a given model, the front tyre matches with the rear tyre as closely as possible.
Many parameters influence this match - notably the profile of the tyre (round, triangular, etc), the rigidity and the characteristics of deformation of the carcass or the operating temperature. This is why it is preferable to avoid mixing tyres.
This is not, however, an obligation. If you mix your tyres and mount tyres of different types, try to comply with the following rule: put the softest tyre at the front. It is, indeed, vital to ensure the best possible grip for the front axle system. A rear wheel skid can be controlled, whereas a front wheel skid almost always leads to a fall. The amount of a tyre's wear is measured by the tread wear indicators in the tread. When the tread wear indicator on the tyre reaches the height of legal minimum rubber (1 mm), it is advisable to change the tyres before the legal minimum is reached (refer to "Running-in and Wear").
It is also advisable to regularly check that the tyre is not damaged, notably by the presence of sharp objects such as screws, nails or pieces of glass.