No DKIM record

you can use any dkim default instruction/tutorials for generating key's, cwp is using centos default in this case.

If you have two servers for sending e-mails you will need a dkim record for each one.

if you have 2 server, u can use the same selector name in the 2 servers, because dkim validation use the server name or ip to validate, selector is just reference.

server 1 selector default

is different to

server 2 selector default

each one generate dkim key and signs the message, for correct use u need to setup dns propertly and ps records to permit many servers ip or names.