There's no point in putting two fuses inline like that -- you still get a maximum of 20A.
On the other hand, those watts ratings they put on amps are almost always overinflated. I bet you'll be fine with a 30A fuse. There's no harm in trying -- the worst that can happen is that it'll blow the fuse, and then you'll know for sure you need a bigger one.
You can power your PC off the same +12V line, if the cable is thick enough. The PC only takes a lot of juice at startup, when your amp doesn't need it. On the other hand, 30A may be insufficient for both PC and amp (especially if you use an inverter rather than a DC/DC to power the PC), so you may end up looking for a 40-50A fuse anyway. Try a car audio specialist, they're sure to have them.