Hi Wes,
Your reasoning is off, but you came to the right place to get the correct solution (I hope LOL).
The easiest way to get this setup working is if the software you use (you didn’t mention what you are using) supports so-called split mode. With a DJ splitter cable (NOT a regular headphone splitter cable!), you can turn your single 3.5mm stereo jack on your laptop, that now holds the plug to your speakers, into a dual-mono output. You end up with two 3.5mm pseudo-stereo jacks. One will take your speakers, the other your headphones. And you will have (mono) sound out of both sides of the speakers and the headphones.
And presto, you can do the things you need to do, like cue a track in your headphones, while the other is playing through the speakers.
If you want a slightly better (stereo, but also more expensive) solution, you can get a 4-channel (DJ) soundcard (USB), using one of the stereo channels for your speakers (RCA) and the other for your headphones. Real DJ soundcards will sometimes have specific headphone connection (1/4 or 1/8 jack).
Hope that helps some.
.
