It's possible to use flights from two different airlines when booking, just add each flight one at a time. i.e create a trip with two one-way flights instead of a return trip.
Good to know: Two one-way flights with low-cost airlines usually come to the same price as a return flight with one of them. Two one-way flights with regular airlines are usually more expensive than a return flight.
If you encounter any issues when combining two airlines or booking a multi-destination trip, get in touch with us at firstname.lastname@example.org