Remote Entry

We have a 2002 outback. The issue we have is that we have the remote keyless entry remote that came with the outback when we bought it along with another remote that was not original as the original was lost by the previous owner. When we lock the car with the original keyless remote and then try and open the car with the other keyless remote the security system kicks in and will not allow us to start the vehicle. how do we go about getting this non factory keyless remotes to work together so we can use either if the vehicle is locked? This might be a dumb question, but also, when we lock the car with the remote and try and open the car with the key the alarm sounds. Is this correct? Can it be disabled? Thanks for the help in advance.