How can I double the watts on a car amplifier

Solution
You probably don't want to. For both the amp and speakers you should find the RMS power rating and not the peak power ratings. Your components may already be balanced.
You can safely use a 300 watt amp on 600w speakers.
If you had a 600 watt amp the speaker would only be able to play 3db louder which isn't a big difference. There is an equal difference in volume when you go from 1 watt to 2 watts as when you go from 300 to 600 watts. You have to keep doubling the power to get the same increase in volume.