I use a beatstep pro with the SP404-SX. It adds velocity to the sounds, which is nice and you can create some great drum machines with samples and using the BSPs drum sequencer (with velocity as well).
What you could do is set the drum sequencer to channel 10 and synth 2 sequencer to channel 9, thus you can have one melodic sequencer to trigger melodic samples and the drum sequencer to trigger 1 shot samples. You just have to set the sequencer channel on the SP404 to 9, so the first sample slots are triggered by channel 9 (or sequencer 2) and the remaining sample slots are triggered by channel 10 (or sequencer 3) on the BSP.
At the moment, my current routing is as follows:
BSP Channel 1 - sequencing the microbrute
BSP Channel 2 - sequencing the TB-3
BSP Channel 3 - sequencing the SP404SX (1 shot vocal samples)
I make more acid house/ghettoTech style music, but if you want to here what's possible, the link below is a video using this setup. The TR-8 is using it's own internal sequencer, a detailed midi routing and audio routing description is on the youtube description on the link below.
Regards,
Mike
https://youtu.be/g6yT-M41rEU