I currently have a 4 gauge power cable for my 450watts 4 channel amp. Recently, I acquired a 1000watts mono for my sub so I need to upgrade my cable. My question is, it is alright to run another 4 gauge cable rather than to buy 2 or 0 gauge cable (both wires will be using 1 lug on each end anyway)?

U will be fine with one single 4 gauge

Or if you are worried, 2x 4 gauge cables will be twice as fine.

What's the # of amps drawn from your 2 amps? The only 4awg cable I looked at specs on was rated at 110Amps, so that should give you an idea.

Me personally would just upgrade the current wire, you will be fine with 2ga wire
You could run another 4ga power wire if you want, but you'll need another main fuse in the engine bay

So I need 2 fuses for two wires, can I just make the two wires goes in one fuse holder and make the fuse 200amps?

I wouldn't as your putting more strain on the single wire
You will also loose alittle bit of current

In that case, I might just take your first advise...get a 2 gauge wire. Thanks

Dude add up your fuses.

Also depends on the amp and how much stress you place on it ( i.e. 2ohm / 1ohm loads ).

Lastly you should assess the length of you wire, the longer it is, the bigger it needs to be. :D