Two different google tag manager accounts in same domain

Question

I have two google tag manager accounts one mine and another of third party website and I need to put these 2 in this third party website. I tried put two gtm scripts after body

Answer 1

Google tells us that you should not use two tag managers on one site. Google Support:

Multiple installations of Google Tag Manager detected

We suggest you place only 1 instance of the GTM snippet on a webpage. Multiple GTM snippets don"t work well with each other
Multiple GTM snippets don"t work well with each other because of which the tag added via GTM may not always fire correctly.

Place only 1 instance of the GTM snippet on a webpage

https://support.google.com/tagassistant/answer/3207128?hl=en#multiple_tags