But you really don't want to do this. As most said, a switch is what you really want.
I worked in a place that used a ton of these ~16 years ago. We had limitations on "dumb" switches due to 802.1x wired port authentication and these made sense because we only had 100Mb switches anyway.
A few manufacturers do make what you asked for: https://www.officedepot.com/a/products/943638/StarTechcom-2-to-1-RJ45-Splitter/#Description
But you really don't want to do this. As most said, a switch is what you really want.
I worked in a place that used a ton of these ~16 years ago. We had limitations on "dumb" switches due to 802.1x wired port authentication and these made sense because we only had 100Mb switches anyway.