|I took another look at your manual, page 16 of the pdf file (not of the manual itself) - I had it wrong, or at least confused - since you have only three 1/8" jacks you do it this way:|
"So are you saying that the only way I can use both my headphone and my speaker at the same time is if I get the splitter"
No - that is just one option.
If you want to do that, normally you would set your software to 2 speaker mode, and connect the Y cord to the Lime (green) jack (Line out, or Spkr if it's an amplifed pair). (In 2 skpr mode, the Light Blue jack is in Line In mode)
You might as well try what is already there before you bother to get a Y cord - Set the playback to 4 speakers. Plug your pair of speakers into the Light Blue jack (rear spkrs in 4 speaker mode), plug your headphones into the Lime (green) jack (front spkrs in 4 spkr mode).
You could extend whatever jack you plug the speakers into (depending on which mode playback is in) to the front of the case if you like.
If they are still very uneven in volume, you may or may not have separate volumes for front and rear in your mixer to choose from - if not, try the Y cord and the settings and connections for that - they may still be uneven in volume.