Audition Ettiquette

Hey there! Just wondering on the etiquette on auditions. Should you audition for everything, even if you can't commit to both (i.e Best case scenario, you get offered both and they film around about the same time...) Do you only go for the one you're most interested in? and then kick yourself when you don't get it and wished you'd gone for the other one? And also, if you do go for both (and get offered both) Is there a polite way to turn down one of the roles? Much advice needed, all appreciated! Thanks