Hertz rental - is it ok to pick up car 2 hours earlier than voucher says?

This depends on the availability of the vehicle at the rental location.

If you have ordered something uncommon then they may have made special arrangements for your vehicle to be there at the scheduled time.

It is best to call the location directly and ask.

I'm a little more optimistic than most other answers. Now, your rental will start early and hence end early, but at a major airport location where there are always a few cars, I don't think you will have an issue. This won't hold only if you are getting a special car or it's a holiday time where cars are scarce. Happens, but seldom.

Flights often land early or late.

Also, the car could only be given back a few moments before, so not available for getting it earlier. It happened to me once, the car just came back, and had to get a maintenance, so i had to leave later than what was booked. So, you don't lose anything by asking, but it is quite possible they won't accept

The turn around of cars can be pretty quick. I returned a car to Avis in Atlanta airport recently. On my way to the terminal, I realised that I had left something in the car. I got back to customer service 20 minutes after returning the car and it had already gone out again. I had returned the car more than one hour early. This indicates that there is no specific car allocated to you in advance, you get whatever is available in the band that you paid for. On a lucky day, there is no car in your band and you get a better car at the same price.

As others say, call and ask but my guess is that it is likely but not guaranteed that it will be okay. I am moderately sure that I have done it sometimes.