For purposes of applying reduction-in-force procedures under subsection (a) of section
3502 of title 5 with respect to positions within the Bureau of Indian Affairs and the Indian Health Service, the competitive and excepted service retention registers shall be combined, and any employee entitled to Indian preference who is within a retention category established under regulations prescribed under such subsection to provide due effect to military preference shall be entitled to be retained in preference to other employees not entitled to Indian preference who are within such retention category.