-overlap them by 4" (so you'll end up with 44" not 48")
-Use fiberglass tape coated in epoxy around the overlap to make the joint. You may want to slightly scuff the carbon fiber first -- someone else will know if that's correct. Any hobby shop (the kind that sell R/C airplanes) will have the materials and show you exactly what to do.
Unless of course you can find the perfect section of telescoping carbon tube to fit inside or over your 15mm rods...
I would not bother with brass or neoprene, as they are both heavy materials (esp. brass! why not lightweight aluminum at least!). If you are going to make it heavy, then use aluminum tubes and not carbon fiber to begin with.
SM-
Thanks for your feedback. My original post wasn’t specific enough, apologies.
I need a setup that allows for temporary connection of the 2 pieces to make a 4' boom, then breaks down into 2’ sections for travel. My use case is rock tour, often including air travel, so I need compact and light gear that sets up/tears down fast and easy.
I currently use a 15mm aluminum rod system, cheese rod, 15mm clamps and 4 shock mounts. My goal is to reduce weight and complexity by moving to carbon fiber rods, a 3-D printed mount, and many fewer clamps & shock mounts.
The expansion nuts weigh 3g each, negligible in the overall picture.