Place a cannon computer. We'll call it Cannon #1.
Add as many cannon weapon blocks for Cannon #1 as you would like.
Place a second cannon computer. We'll call it Cannon #2.
Add cannon weapon blocks for Cannon #2, but not more than you have for Cannon #1.
Mouse over Cannon#1 and type "c". This will select the computer as the command block, and you will see it bordered in an orange wireframe.
Mouse over Cannon#2 and type "v". This will make the computer a slave to Cannon #1, and you will see it bordered in a blue wireframe.
Every cannon weapon block that you added to Cannon #2 will increase the fire rate of Cannon #1, but it will also reduce the cannon's damage per shot. Adding more blocks to Cannon #2 than you have in Cannon #1 will not improve it further. Also, Cannon #2 and it's weapon blocks can be anywhere in your ship - they can even be split up in different groups. The game only wants to know how many blocks Cannon#2 has in relation to Cannon #1.
Hope that helps.