Is there any difference between regular and "split" hardpoints?
No, there is absolutely no difference in terms of damage etc. The only difference is cosmetic in that two (smaller) projectiles show in battle.
But good question as I remember having the same question when I played.
If I remember correctly, I read this somewhere on the GSB website or forum, but that was quite a while ago, so it would be difficult for me to come up with a link.