These are indeed links but quite random ones. Mostly they are reporting that growing two things together in a field can give higher yields than growing a single thing. None seem to be related to GMOs, which could obviously also be grown with complimentary species to increase yield.
All crops are inclined to be grown in monocultures. That's because even if you'd get more yield pairing with other species — as in these studies — pragmatically the time and cost of sowing, harvesting and separating multiple species outweighs the theoretical yield gain per unit area of a monocrop.
