Can I filter the Output data separately?!

Elastic Stack Logstash
5

You should be able to use the clone filter3 to generate one event per output and format these differently.

Ah, forgot about clone. Yes, that's the way to go.

[...] but how do I tell Logstash to send the original event to 'orders' index while sending a copy of the event + custom fields to the 'orders_alt' index?

The filter's clones option sets the type field of the clone(s), so just add conditionals to selectively apply filters and send them to different outputs.

filter {
  clone {
    clones => ["cloned"]
  }
  if [type] == "cloned" {
    # this filter
  } else {
    # that filter
  }
}