I assume that you have:
1. two or more websites
2. with different implementations of GTM (different GTM codes)
3. which you want to act as one entity
Therefore:
1. Create a constant variable, containing theURLs of the websites, separated by ‘, ‘ (comma & space, eg: ‘website-a.com, website-b.com, website-k.com’).
2. In your tags under ‘More Settings’ (or in the general setting variable), enter ‘allowLinker’ at Fields to Set - Fieldname and ‘True’ as corresponding value.
3. Also under ‘More Settings’ open the options for ‘Cross Domain Tracking’. Use Hash and Decorate Forms is standard set to ‘False’. That is fine in most cases. Below ‘Auto Link Domains’, add your constant variable containing the URLs included in your cross domain tracking.
4. Do that for every GTM property / account involved.
5. Make sure not to make chaos which the GA properties and views that you’re sending the tags’ data to.