A process for the synthesis of an unsaturated product by cross metathesis between a first unsaturated compound having at least 8 carbon atoms and a second unsaturated compound having less than 8 carbon atoms, the first unsaturated compound being capable of producing an unsaturated coproduct comprising more than 14 carbon atoms, by homometathesis, said process including at least one production phase which includes: feeding a reactor with the first unsaturated compound; feeding the reactor with the second unsaturated compound; feeding the reactor with at least a first metathesis catalysts, then feeding the reactor with at least a second metathesis catalyst; withdrawing a product stream arising from the reactor; the turnover number of the first catalyst being higher than the turnover number of the second catalyst so as to achieve the same target degree of conversion of the first unsaturated compound.