It is normal to apply through one agency but some people have applied through two and taken the one that offers the first chance at the assessment. Once you agree to attend the selection process then you'll be stuck with that agency.
The principal agencies for this job are CREW and PARC, with Longreach mainly being used by the Qantas pilots on leave without pay. I believe Rishworth simply collect names and then hand them over to CREW and most people start sucking their teeth when Rishworth are mentioned and wouldn't touch them with a bargepole. My preference would be CREW, then PARC.