UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.110 Safari/537.36

Example URL:

Steps to reproduce the problem:
1. Start a chrome from command line with args --user-data-dir=/tmp/chrome-data-1 --disk-cache-dir=/tmp/chrome-cache
2. Open google home page: www.google.com
3. Start another chrome from command line with different user-data-dir but same disk-cache-dir: --user-data-dir=/tmp/chrome-data-2 --disk-cache-dir=/tmp/chrome-cache
4. Open devtools and monitor the network
5. Open google home page: www.google.com
I can see all the images are from disk cache on the second Chrome. This is expected behavior. 
However, When I used ChromeDriver to start two chromes, and repeated step 4 and 5 on both of them, the images on the second chrome are not fetched from the disk cache. 
Below Python code can be used to start two chromes by WebDriver. I'm using ChromeDriver 2.42.

from selenium import webdriver
from time import sleep

options = webdriver.ChromeOptions()
options.add_argument("--disk-cache-dir=/tmp/chrome-cache-wd")
for i in range(1, 3):
    webdriver.Chrome(
        executable_path='driver_path',
        chrome_options=options
    )
sleep(3600)

What is the expected behavior?

What went wrong?
Disk cache cannot be shared by multiple chromes started by ChromeDriver.

Did this work before? N/A 

Chrome version: 70.0.3538.110  Channel: stable
OS Version: OS X 10.13.5
Flash Version: