Etiquette for consulting multiple doctors?

If a patient is under the care of one doctor but would like to consult another, to get more information and ideas (not necessarily to request care from that second doctor), what is the proper etiquette?

Should the patient first broach the subject with the present doctor?

In this case there aren’t any insurance requirements for a referral, though I know in many cases that would decide it. And I know a patient should not request simultaneous care from two different doctors without their mutual cooperation, and that in some cases like getting controlled substances prescribed it may be illegal. This is just a standard etiquette question.

Thanks!