I have a 2000 Volvo and a 2000 Nissan Altima. an anyone recommend a service program that’ll handle codes for both cars? What hardware might I need?
Do you mean a “Service engine soon” light or a Check Engine Light (CEL)?
I don’t think there is anything you can do with a service engine light other than turn it off. Most any code reader will handle the CELs for most cars from that time.
Service engine soon light on my Volvo, 118k needs a serpentine and timing belt.
I don’t know about the Nissan, but I believe the Volvo uses a series of actions by the driver, like turn the ignition off and on three times followed by using the right turn signal and then dancing a jig. 
My point is I don’t believe it requires a tool.
I might suggest if you have a timing belt due, that you do it or have it done a lot sooner than later. I would hope anyone who does a timing belt change should know how to rest the service light.javasailor,
These vehicles both should be using OBD2. A scanner can be purchased for $40 to $80 that will read codes and reset the light. Actron is one company that makes them.