Direct satellite-to-device (DS2D) communication enables standard mobile devices to connect directly to low Earth orbit (LEO) satellites, providing global coverage without reliance on terrestrial infrastructure. However, the DS2D uplink is fundamentally constrained by long propagation distances, severe path loss, and stringent user equipment (UE) power limits, making uplink reliability particularly challenging at low elevation angles and beam edges. This paper investigates the integration of supplementary uplink (SUL) technology into DS2D systems to enhance uplink robustness while preserving UE power efficiency. Leveraging the predictable geometry of LEO satellite orbits, we develop an elevation-aware SUL framework that adapts uplink operation across frequency bands based on elevation-dependent link margin estimates. The proposed approach schedules the UE to transmit on either a primary uplink carrier or a lower-frequency SUL carrier. An elevation-aware SUL activation algorithm with hysteresis is introduced to guide uplink carrier selection while preventing frequent switching. Simulation results demonstrate that the proposed SUL framework extends effective uplink coverage toward low-elevation and beam-edge regions, improves uplink availability over a satellite pass, and achieves stable operation with a minimal number of uplink transitions under realistic UE power constraints.