Hi. Backing tracks, so you mean longer than 1 mn? How do you want to play them?
It seems you are confusing several things.
There are 8 audio tracks, you can play 8 samples max at the same time.
The first 8 trigs triggers the 8 tracks when they are set to PLAYS FREE. 9-16 trigs can trigger default track sample. (It is possible to play different samples with trigs 1-8 using Plays Free Tracks, tell me if interested).
You can trigger all SLOTs samples with SLOT mode (Function + down arrow).
Maybe the easiest to trigger 16 backing tracks with 16 trigs.
In Grid Recording mode, on a same track you can assign a different sample per trig, holding a trig and turning Level knob. Usefull for short samples. Slices can be more interesting in that concern.
Default track samples (trig 9-16) are set in Parts.
You can assign 8 default track samples per Part. 2 Banks use different Part so it’s doable.
Also doable with 2 patterns with different parts.