Good Morning, David,
Optimal "normal" times are slightly different. I use 10 minutes for T-100 and 9 minutes for T-400, both in T-Max developer diluted 1:7 directly from concentrate. With times so close, however, I wouldn't hesitate to process both films together in the same tank for, say, 9.5 minutes, assuming a non-critical situation. With roll film or 35mm, the reel with T-400 film can be put on top in the tank and pulled a minute ahead of any T-100 and dropped in the film washer while the T-100 continues development for another minute. Most of the time, I will slightly adjust the time for either film depending on the subject contrast.