I regularly do two tanks at a time, and have done three at one time. I find two manageable, three is doable, but can lead to mistakes for me.
When I first started doing multiple tanks, I made a chart for each tank and staggered the development. Most of the time, I will start the second tank 3 to 5 minutes after starting the first. By doing this, I make sure that One tank will only need agitating while I am changing chemicals in the other tank. I can pour out and add chemicals within thirty seconds, so changing chemicals in one tank does not interfere with agitating the other tank.
I use two timers. One timer counts down to zero and the other starts at zero and adds the time. I found this helps to keep me from getting confused. But it is not necessary. But two timers definitely makes it easier.
If I miscalculate and find i have to change chemicals in both tanks at once, I always make sure that the developer time is correct. If I have to leave one film in the fix for an extra 30 seconds, so be it.
I also make up all the chemicals for both tanks before starting and have the beakers separated on the counter, the chemicals for tank one on the left, the chemicals for tank two on the right.