Adding a second battery with an isolator is an option to power radios, but for a pair of scanners, that's kind of overkill. Might make sense if you were running several transceivers and a considerable amount of lights.
For a couple of scanners, you could do a few different things…
1. Take a power feed off the vehicle battery, run it through a relay that is triggered by the engine ignition. This will power the radios when the engine is running and shut off the power when it's not. Drawback is that you have to have the engine on for the radios to be on.
2. Take a power feed off the vehicle battery, run it through a relay that is triggered by the vehicle "Retained Accessory Power" (RAP, that's a GM term, but most cars have similar). The radios will be powered when you start the car, and will remain on even with the engine off, until you open a door or it times out.
3. The more common option in the radio industry is to use a timer device triggered by the ignition or RAP. Power from the battery is fed through the timer device to the radios. When the engine is started, the radios power up. When the ignition or RAP is shut off, the timer starts and the power will be disconnected when the times is up. Usually the timers can be set anywhere from a few seconds to several hours.
Option 3 is what you'll usually find in police cars. That way the radios shut down and the battery is protected. Actually, in some cases, there is an additional battery in the trunk (Crown Vic's and others) that charges off the alternator, but is isolated when the engine is off. That battery is often used to power the radios, lights, computer, etc. It going dead won't stop the car from starting.
For a few scanners, you could run a power feed off your battery to a Lind shutdown timer or Havis ChargeGuard type device, then to a fused distribution block near your radios. The shutdown timer/chargeguard would be triggered off ignition or RAP. That way your radios would have power when you start the car, but would power would be shut off after a time you set.
Here:
https://lindelectronics.com/product-category/shutdown-timers/
Havis Products | Auto Shut-Off Timers
They are pricy, but if you surf over to e-Bay, you can often find them in the $30 or less range. I've picked up a few for personal use for $20 or so.
As for the power distribution part:
https://www.amazon.com/MICTUNING-Il...1-spell&keywords=ATO+fused+distribution+block
https://www.amazon.com/Blue-Sea-Sys...3-spell&keywords=ATO+fused+distribution+block
https://www.amazon.com/ZQUS-GZQ-8-W...8-spell&keywords=ATO+fused+distribution+block