Should water heater thermostats be set to the same temperature?
Setting the top element slightly higher helps the elements to be used evenly.  If you set them to the same temperature, the bottom element will be the first one on every time, and frequently the only one used to maintain the temperature.  If you set the top element too much higher, then it will be used most of the time, and the bottom element acts more as a pre-heater.

Having them used evenly will spread out the minimum time to replacement.  But I wouldn't go through too big of an effort to change the settings on an old water heater.  Whenever your elements do go, or whenever you decide to change the temperature, that would be the time to make this adjustment.