It is possible to connect multiple odometers to a unit. To add a odometer, first select a unit, click on Edit and then click on the Odometers-tab. Give the odometer a name and click on Add odometer, after that you need to enter the current value, which is the current mileage on the unit, finish by clicking on Save.
Now, a new odometer is connected to your unit and you can see that the new odometer reading has appeared besides the standard one on the overview of the unit. Now you can also connect services and checks in the checklist to a specific odometer.