OK what's happened is you are now sharing information because of having multiple Google accounts on the same device. I had a similar issue after logging into my kid's accounts on my device to search for their lost phones using Find My Device. After finding the device I never removed the accounts from my phone so when their mom calls, the caller ID will usually say Mom and not her name as I have it listed. Also all of their contacts show up in my Contact list which duplicates for contacts we share and just shows up for the ones we don't. They can't see my contacts or any of the names I have for them because my account isn't on their device but because their account is on my device I see all of there contact. Since WhatsApp, like other messaging apps, draws from your contacts it's going to see whatever is listed there.
I'm not fully understanding all of the ins and outs of your situation so I can't really tell you how to fix it other than removing one of the accounts from the device.
You could try turning off the contacts for the unwanted account on the device so they aren't visible in contacts but I'm not sure that will take care of the issue 100%. It may just hide them from view.