There are some discrepancies in what you say and the Icinga ecosystem structure, so lets clarify those and then answer your question:
Icinga2-Web2 reads,can updates some attributes and display the information that is stored in the IDO engine ( be it Postgres or MySQL) .
Icinga writes the data to the IDO.
So with that understanding, Yes you can connect multiple Web2 instances to the Database, but it is a good idea to ensure you are not creating a situation that will cause 2 Web interfaces trying to update the same records.
The way to do that is to create 2 permission roles on the DB, one for the full access and another with “read” only so it can be a simple dashboard and thus negating the possibility of conflicting commands coming from the Web.