You need to use an old Catalyst driver package that supports it. In this Hardware Secrets article they used 15.7
As a commentary on the usefulness of these old technologies, nVidia had Hybrid SLI, ATI offered Hybrid Graphics, which AMD renamed Dual Graphics, none of which offered much benefit and why they were eventually discontinued. Pairing an underpowered IGP with an equally underpowered discrete card resulted in minimal boosts of typically 10-20% higher than using only a single graphics solution.
Ryzen R7 5700X | B550 Gaming X | 2x16GB G.Skill 3600 | Radeon RX 7900XT
Ryzen R7 5700G | B550 Gaming X | 2x8GB G.Skill 4000 | Radeon Vega 8 IGP
Ryzen R5 5600 | B550 Gaming Edge | 4x8GB G.Skill 3600 | Radeon RX 6800XT