Catalysis of the reaction: ATP + 5-diphospho-1D-myo-inositol (1,2,3,4,6)pentakisphosphate = ADP + 5,6-bisdiphosphoinositol-1D-myo-inositol (1,2,3,4)tetrakisphosphate.
inositol heptakisphosphate 6-kinase activity