Channel Islands airlines Aurigny and Blue Islands have been given permission to code-share on the inter-island route.
The airlines plan to use only Blue Island's planes to fly the Guernsey-Jersey route but both will take bookings for the seats.
Channel Islands regulatory authorities gave permission but imposed conditions.
Each airline has to market seats independently and Aurigny must tell customers Blue Islands is the operator.
Channel Islands Competition and Regulatory Authorities Chief Executive, Andrew Riseley, said he gave permission as the airlines had both said competition would go without it.
He said: "On balance our judgement is that the best means of protecting customers is to ensure that competition on the route remains as vigorous as possible and the conditions we have set for agreeing this exemption should achieve this."
Under the agreement, which will run for two years, Aurigny will buy a fixed block of seats on flights operated by Blue Islands.
The airlines will fly six or seven flights in each direction on weekdays, four flights in each direction on Saturdays and three flights in each direction on Sundays.
Mr Riseley said responses to a consultation focused more on quality of services and level of capacity rather than fare structures.
He said the airlines had been told they must increase capacity where it is necessary to meet demand.
"The airlines have provided future forecasts under the joint operation which show financial improvements for both parties.
"[We] are of the view that, without this financial improvement, one of the parties would exit the route altogether."