Pine Island and Thwaites Glaciers, situated in the Amundsen Sea Embayment in West Antarctica, are the two largest modern-day contributors of sea level rise from Antarctica, and their potential future retreat threatens the stability of the West Antarctic Ice Sheet over coming centuries. Here we present numerical simulations exploring the sensitivity of both glaciers to reducing the basal friction (bed-weakening) in response to ice thinning and the rate of ice shelf melting over millennial timescales. We find that bed-weakening leads to faster rates of retreat and sea level rise when confined to the grounding zone, but expanding the region of bed-weakening delayed retreat instead. Pine Island Glacier is highly sensitive to increases in the ocean melt rate, while Thwaites Glacier is more sensitive to bed-weakening. The sea level contribution from Thwaites Glacier over the next century is particularly insensitive to changes in ocean melting. Despite the relatively weak buttressing from the Thwaites Ice Shelf, it still provides sufficient support locally to suppress the rapid retreat that occurred with the complete removal of all floating ice. These results highlight the sensitivity of sea level projections to the relatively poorly known basal effective pressure, and the need for future ice flow models to be coupled with with highly-skilled basal hydrology models.