You can install RescueTime on multiple devices

There is no limit to how many computers or mobile devices you can install RescueTime on, however, your reports will not be differentiated by the device. Your data will be aggregated. 

You can find more details about the operating systems compatible with RescueTime here. 

The accuracy of your reports shouldn't be affected by multiple computers installed unless by some chance you are using two computers simultaneously. In this case, you may see more time logged than expected. RescueTime reports only active use on the computer window. We share more details about how RescueTime records time.