Ski boot liners are designed to fit as closely to the foot as possible while being comfortable. Ideally, you’ll want as close of a connection between your body and the boot as is you can. Wearing more than one pair of socks quickly leads to sweat, chafed feet and hotspots where the socks bunch together inside of the boot liner. It’s always better to opt for a single thicker pair of socks to fill dead space in the boot or add an extra boost to insulation.