How to run two work groups per one compute unit on AMD GCN cards

Question

Usually one compute unit can only run one work group. But AMD's doc says there can be more than one wavefronts running on the same compute unit. How can I do that? Is that an OpenCL function for that? Or I need to use assembly instruction? I want to do this because my work group size is 20 and I want to run 2 work groups per compute unit, so that each group can use 32 KiB LDS (64 KiB total per CU, each wavefront can use up to 32KiB so I want to run two wavefronts to use the full amount of LDS).