I looked up bike cable extenders such as:
https://www.ebay.com/itm/181288873909
They are expensive, but would require you to cut the existing cable end to insert.
Another option is from porkchop BMX-
https://www.porkchopbmx.com/porkchop-bmx-porkvert-bicycle-gyro-upper-brake-cab.html
This is a 2 into one splitter, but I bet it would work just using one to one.
Now, what you'd have to do is dremel a slot into one of the 3 holes on the naked aluminum piece so you could slide in your existing cable without cutting the end, but it should work, and is the cheapest thing out there. All other cable spltters seem to be in $50 dollar range.
This is another splitter that would accomodate the cable end. It is a 2 into one, but would probably require you to dremel the outside of the cable extender housing to fit your cable . Also $$$
https://www.xvkdfashion.shop/?path=page/ggitem&ggpid=2426868
Personally, I'd go ahead and cut the round ball off the end of your existing cable, or just get a longer cable. Sorry I couldn't find a direct plug and play. These all require fidgetting. Perhaps someone else will have a better idea.
Other random splitters ($$$ and would still require cutting cable)
https://www.jtekengineering.com/products/double-control-model-l/
https://problemsolversbike.com/products/cable-doubler
At this price point and rednecking level, it might make sense to get a longer proper cable.