Factor pertianing to geogrpahical location like datum hight from mean sea levels, which affects the rate of flow and storage capacity, uncertainty of the number of potential failures like earth quakes, floods apart from environmental impact are to be considered for designing of mulitple dams on same river.
management of sedimentation scouring from the upstream dam, versus re-deposition in the downstream one.
Environmental effects of a widely varying flow regime between the two dams, compared to a (possibly) less wide range of flows downstream of the lowermost one.
The management of both, to optimise the hydropower generated to the national grid, at those times when the power is most valuable, while operating them within safe upper/lower water levels