How many PCIe lanes does my second GPU actually need?

Far fewer than people assume. For Malairte mining, a GPU does most of its work on its own onboard VRAM, so the PCIe link to the motherboard carries relatively little traffic once mining begins. A second card running at electrical x4, or even x1 through a riser, will mine at essentially the same rate as one in a full x16 slot. The slot width matters far more for gaming and large data transfers than for the steady-state mining workload. This is good news for home builders, because it means you can use a motherboard whose second physical x16 slot is wired to only x4 lanes, which is extremely common on consumer boards. The one thing to confirm is that the slot is physically long enough to seat the card or that you have a quality riser to reach it.