You can stack as many instruments as you want on GS3. If you use two instruments that don't overlap, like tuba and flute, you will have a true split. If you use french horn and bassoon, you will get a mix on some keys.
You could use the editor to remap the keys (regions) for some instruments, in case you wanted to create very specific mappings and octave changes without overlaps.
It's easier to do on a keyboard with splits, but can be done with Giga. Once you've got a setup that you like, you can save the stacked instrument as a *.gsi file, and then you can call it up instantly. So, it's more work up front, but can work very efficently, once you have a good setup.