Let's take the basic example. You have 2 Dorsal Mars-Pattern Macrocannons that you elect to fire at the same target. You choose to fire them as a "salvo", granting you a +10 bonus to hit. With a Ballistic Skill of 70, you roll a 30 to get 4 degrees of successes, or 5 shots total. Normally a Mars-Pattern Macrocannon only has Strength 3, but because you are firing two in a salvo, you combine their totals and can thus theoretically score up to 6 hits.
Your enemy's void shields absorb 1 shot, but you still have 4 shots get through to cause damage. You then assign these 4 shots to the weapons used in the salvo (irrelevant since they're the same) and roll (1d10 +2) four times. With average rolls, you get a result of 30 which subtracting the target's armour of let's say 12 results in a total damage of 18 from your salvo.
Absolutely key point: The number of shots that hit are still equal to Success + Degrees of Success. You do not double or triple the number of shots just because you're firing in a Salvo.