Notice that, depending on your motherboard, there may or may not be such "competition" for lanes. In typical consumer boards, you may have 2, sometimes 3, physically x16 slots that share lanes and switch between x16 or x8+x8 (or x8+x4+x4 in the 3-slot case) depending on what you plug. As @Lurick said, such lane split will occur the moment you have something plugged in into the additional slot(s).
However, there are additional slots that are wired independently and don't trigger any lane re-config (typically x1 slots in consumer boards, but sometimes some bigger slots as well, or x16 slots in "HEDT" motherboards). In those cases there is no change in the lane setup. Furthermore, you have PCIe slots that are routed through the chipset and share a common connection to the CPU. In those cases, the lanes don't change either, and plugging stuff in itself generates no conflict, but they will "compete" for bandwidth when in use.
- for independently wired slots, it doesn't matter
- for shared CPU lanes with switch/bridge, what matters is plugging
- for shared CPU lanes through the chipset, what matters is activity/usage