Doing both Spiti and Ladakh at the same time. Great. Good thing about this plan is that you would not need to stay at Sarchu for acclimatisation as you'd be traveling from Spiti which is at great altitude itself. If you need any help with planning, let me know. I've done this route and I organise tours for Spiti and Ladakh now, so I take myself well qualified to answer your questions! Good luck and Happy travelling!