useing one device on midi and another on usb are going to give you latency issues. you could also try daisy chaining the devices off one midi port.
ok i searched out the specs on all your gear, and here is my suggestions, ill give you 2 possible serarios, depending all on cubase. i havent ever used it so i dont know is they will both work, but one will.
#1: use the mpc as the master. Slave the triton and cubase to it.
use the mpc to sequence and cubase to record the audio, you could also have cubase record all midi data to if you wanted.
EX: MPC midi out(master)->Triton midi in-> Triton midi out->mpc midi in
Triton midi threw->cubase set to record all 16 midi channels.
In this case all sequenceing would be done on the MPC. when you hit play to record into cubase, midi data would flow from mpc to the triton, a copy is sent from the triton threw port to cubase, and the triton midi out sent to the mpc(maybe you dont need this during recording, but you do when sequenceing). the tritions midi threw copy sent from the trition to cubase is recorded. this way you can export the midi data later back to the mpc just incase.
#2: Leave cubase as the master and slave the mpc and triton to cubase.
that Emu card has 2 midi in ports and 2 midi out ports. why use usb on the triton? midi the mpc on one port and the Triton to the other port. that way you can use all 32 avalible midi channels on the card.
EX: Cubase(master)->emu midi out 1 ->mpc midi in->mpc midi out-> emu midi in 1. Then, Emu midi out 2 -> triton midi in-> triton midi out -> emu midi in 2.
this way when you hit record and play in cubase the mpc and trition should playback in sync via timecode. cubase would have recorded the midi notes, and just sends that data to the devices, and from the devicescomes the audio playback like normal.
useing the usb on the triton for much more then patch editing and data transfer is silly if you have another midi device useing a normal midi cable, it will throw the timecode off for sure.
anyways, try these out, i dont know if cubase can be a slave to another device or not, but if it can, i would use thempc to sequence, and cubase to record the audio playback and midi data for recovery reasons.
Peace,
Rob