Condensation of UDP-2,3-diacylglucosamine and 2,3-diacylglucosamine-1-phosphate to form lipid A disaccharide, a precursor of lipid A, a phosphorylated glycolipid that anchors the lipopolysaccharide to the outer membrane of the cell. a lipid X + a UDP-2-N,3-O-bis[(3R)-3-hydroxyacyl]-alpha-D-glucosamine = a lipid A disaccharide + H(+) + UDP Bacterial outer membrane biogenesis; LPS lipid A biosynthesis. In the C-terminal section; belongs to the LpxB family.