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this might sound a bit obvious, and without getting too technical, speakers make sound by pushing air.
two ways to do that: 1) size/quantity of speaker(s), and 2) how far the speaker cone moves out/in.
You can make a rough guess as to sound volume by simply comparing sizes.
one 6X9 = 42 sq in, so two = 84 sq in
one 8" = 50 sq in, so two = 100 sq in
one 10" = 79 sq in, so two = 158
one 12" = 113 sq in, so two = 226 sq in
The other component of should volume is the movement capability of the cone (usually referred to as xmax). This varies wildly, and is frequently, but not always, a function of quality.
A recommendation would require knowing your budget and amount of trunk space you're willing to part with.
As to wiring the amp, one thing to check would be to see the size of the power cable supplying your current amp and determine if you could use a distribution block to split the wire to both amps.
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