15 nm-Co/15 nm-Ni/p-Si (100) was thermally annealed using rapid thermal annealing for 40 s at $700{sim}1100^{circ}C$. The annealed bilayer structure developed into the composite $CoNiSi_x$ and the resulting changes in sheet resistance, microstructure, and composition were investigated using a four-point probe, transmission electron microscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. The final thickness of the composite $CoNiSi_x$, was approximately 100 nm and it maintained its sheet resistance below $5{Omega}/sq$ after silicidation annealing at $1100^{circ}C$. The proposed $CoNiSi_x$ silicides may be superior to conventional single phased silicides due to their improved thermal stability and thickness adjustment.