The question is, does it even matter that much, or is the differences negible? Of attacking a single time without splitting or splitting and attacking same target.
Attacking the same target twice vs once means:
- half dice pool with each attack => half the expected hits each attack
- defender rolls defense twice => less overall net hits => less chance of even hitting + less damage added to the damage rating
- defender rolls armor+body twice => mode damage reduction
- Attacker adds base damage of the gun twice => well more damage, but that might get eaten up by the former two points
So just an easy example (assuming average 1/3 chance of success).
Attacker: 24 dicepool
Defender: 9 dodge
Damage: 8P
Armor+Body: 18
(I'm just making up numbers, not having anything special in mind)
One shot:
Attacker Expected hits: 8
Defender exp. hits: 3
Net hits 5:
Damage: 8+5
Armor roll: 6
Total damage: 7
Two shots:
First shot:
Attacker hits: 4
Defender hits: 3
Net hits: 1
Damage: 8+1
Armor Roll: 6
Damage total: 3
Second Shot:
Attacker hits: 4
Defender hits: 2 (since his dicepool is 8 now)
Net hits: 2
Damage 8+2
Armor roll: 6
Damage 4
Total damage 3+4 = 7
Doesn't look a lot different. It might be different using other numbers, as I said I just pulled them out of my ass.

Now it might be different with Full Auto too, but I'm not sure a "20 round burst" would be worth it, since I have seen very few people that even spot such a high defense score that it would matter.
I also know the math used is not too solid, as I should have used something like "chance to hit at all" or something and not just "average hits". (In the 2-shot example it's alot easier for the defender to avoid all damage by just rolling 1 success more or the attacker 1 less. In the first example, that is a lot harder)