It doesn't work like that. If you cancel the second flight, the ticket will be repriced as a single, its in the ticketing rules

800 single
300+900=1200 total

Cancel return leg, ticket is repriced as the single
800
800-1200 -> 400

So 400 refund, so no actual benefit, which is either random or someone in head office has been careful to design the pricing to prevent this kind of game strategy