I've been playing around with GPO in FL Studio and have the following tips to enable multi out:

- Fruity wrapper - under Compatibility - make sure the "use fixed sized buffers" in not checked

- Fruity wrapper - make sure "enable multiple outputs" is checked

- Assign the FX output in the wrapper to a mixer channel

- using the main Kontakt VST (not the 8 or 16 output version) assign each instrument to a different channel in the Kontakt mixer

- Click on "conf" for each GPO mixer channel and assign channel 1 to the first two outputs, channel two to outouts 3 and 4 etc.

hope this is helpful