Amy and I did IMKS and HighCliff HIM on back to back weekends. It depends on your goals. I pushed the first one prety hard and the second one was more for fun. Mainly just rested between races, but was on vacation.
It actually worked well..except for Amy's trip to the ER after the second race
I've done this before as well as doing a HIM in the middle of back-to-back 40 hour weeks (during www.EpicCamp.com). I've also participated in the Triple T, which has two Olys (Sat) followed by a HIM (Sun). If you are fit, and extremely durable, then it can be done safely.