If you want to compare data of different accounts or combine them in Webtrekk, you need a so-called global account.
The prerequisite for this solution is that the custom parameters and categories in the individual accounts match.
Using "Data Profiles", the requests sent to the individual accounts can simply be duplicated in order to store them in the global account.
If the parameters are configured differently, the data would have to be sent directly to Webtrekk via two pixel instances, each with a different assignment. But this is usually associated with higher costs and maintenance effort.
If the websites, which are to be combined in the global account, are on different domains, but the visitors are to be evaluated across websites, you need cross-domain tracking. The easiest way to achieve this is to use 3rd party cookies, thus, simply placing the cookie in a common track domain. However, the acceptance of 3rd party cookies is significantly lower depending on your browser and settings.
We generally recommend the use of your own track domain to improve data quality in general. This will prevent most AdBlockers from blocking your requests. In the case of cross domain tracking, you should create the track domain in the domain of the largest account. Thereby, the cookie is a 1st-party cookie for this account and only for the others remains the issue of cookie acceptance.
See also: Custom Track Domain