Normally if you ride a lot you will wear the center tread section off the rear tire first. Most people then just swap the front & rear, and wear the center off the other tire before buying new ones.
If your tires are dry-rotting it might be wise to replace them both just for that reason.
Also make sure you're not storing them in bad conditions--some common ones being in direct sunlight, near oil or fuel vapors or near an electric motor that runs a lot.