Welcome to the forum.
There are many considerations you have not commented about, so it is hard to answer. Which dash cam are you considering? Why are you reluctant to use a second power socket? Do you need to remove the cameras when you park the car? Is it okay to use a fuse tap to provide hidden extra power sockets? Does the power sockets in your car provide 12V all the time for Park mode, or only during driving use? Is that okay for your need?
You may be able to use a USB "Y" cable to split the power into two cords. You may need some female to male connections for this. This Y cable method could have lots of issues though. The camera power requirement is different among many of the dash cams so one answer does not fit most cams. Some are 12-14 volt direct to the camera and have no electronics in the power plug, so they could be spliced together or Y adapter powered without issue. Many use 5 volt input at the camera, so their power plug has 12V DC to 5 V DC conversion. These you would have to be careful as the circuitry may not have the capacity for two cams. I think most users would encourage you to use both original power cords and find a way to connect to two power sockets, or hard wire both to 12 volts if you have the option to do this.
By the way, I think two separate cams allows you to pick exactly the features you want for front and back. I do this for my families cars. There are a couple good dual lens cams available, only the better models work well at night. Be careful on picking something you like. Explore this forum before you buy. It is a great resource for user experience with the many models.