> The first question is how a define a second user?
You can register another user to the CA and use that user for the second PC.
> how should I change the test-network/configtx/configtx.yaml file to set that only the one of the two users have the ability to change the owner of a car and the other one user is just a reader?
You should change the source code of fabcar chaincode instead of changing the configtx file.
I recommend you to check out papercontract.js chaincode in the commercial-paper sample. It will be more practical to separate the organization of the two users if you want to give different access rights to the users. papercontract.js uses ctx.clientIdentity.getMSPID() to check the msp id of the client user identity.