@the.rampage.rado
I never use Advanced Stock Management in the shop; I believe it’s a feature that causes nothing but problems and offers no benefits whatsoever. So I haven’t tested "split orders" with ASM.
Yes, different carriers must be configured for the products for "split orders" to take place.
Yes, with the module I’ve attached, you can change the carriers for individual sub-orders in "split orders" mode.
And, as a point of interest, a Prestashop fork called PhenixSuite has fixed this.