The Ryzen 5600G only supports PCIe gen 3.0
The Radeon RX 6600 only uses x8 PCIe lanes.
If it was limited to only one or the other (either x16 of PCIe 3.0 or x8 of PCIe 4.0) you'd be fine, however combining both the graphics throughput will be restricted to less than 8GB/s, performance will take a hit.
Recommend you pick a GPU that uses all x16 lanes or plan on upgrading the CPU to one that supports PCIe 4.0
Ryzen R7 5700X | B550 Gaming X | 2x16GB G.Skill 3600 | Radeon RX 7900XT