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. 2-N,3-O-bis[(3R)-3-hydroxytetradecanoyl]-alpha-D-glucosaminyl 1-phosphate + UDP-2-N,3-O-bis[(3R)-3-hydroxytetradecanoyl]-alpha-D-glucosamine = H(+) + lipid A disaccharide (E. coli) + UDP a lipid X + a UDP-2-N,3-O-bis[(3R)-3-hydroxyacyl]-alpha-D-glucosamine = a lipid A disaccharide + H(+) + UDP Glycolipid biosynthesis; lipid IV(A) biosynthesis; lipid IV(A) from (3R)-3-hydroxytetradecanoyl-[acyl-carrier-protein] and UDP-N-acetyl-alpha-D-glucosamine: step 5/6. Belongs to the LpxB family.