support of multiple databases on different hosts by pgpool2 Let's assume We have 2 databases:
Database1 is located on host1 and host2
Database2 is located on host3 and host4
If I'll use pgpool2 to connect to these 2 databases with single pgpool config for backend configuration:
 backend_hostname0 = 'host1'
 backend_port0 = 5432
 backend_weight0 = 1
 backend_hostname1 = 'host2'
 backend_port1 = 5433
 backend_weight1 = 1
 backend_hostname2 = 'host3'
 backend_port2 = 5434
 backend_weight2 = 1
 backend_hostname3 = 'host4'
 backend_port3 = 5435
 backend_weight3 = 1

With this kind of configuration, will pgpool2 throw an error in case of trying to connect to Database2 for example which is absent on host1 and host2 or it will determine easily necessary hosts which contain appropriate database with error and coordinate client in appropriate way to connect to it?
Why I am asking this, because I don't see that pgpool supports something like conf.d directory where I can put separately hosts with different databases in order to connect to them via clients. Instead I see support only of single config file which is pgpool.conf for backend definition.
no. With this kind of configuration, pgpool2 will not throw an error.
Pgpoll configuration is much more complicated than that. I used to think it as easy as you list config. Later, I just follow the full procedure.
Here for your reference.
https://www.pgpool.net/docs/latest/en/html/example-cluster.html
Good luck