Re: dojo head bans students from training with another teacher (ex student)
I understand that the notion of "sensei's dojo, sensei's rules" is relevant here. At the same time however, your "sensei" has already tipped his hands regarding his capacity to step outside of himself (i.e. "it's personal"), and, in my opinion, no matter how Japanese he is, no matter what his rank is, no matter what his title is, no matter how many years he's trained for, no matter how spiritual he may seem to be, no matter how powerful and martial his technique appears, his Aikido can only be of a limited (lesser) quality. He may be the best bet in town, the best you've seen, so you may have to add that into your decision-making process, but you certainly shouldn't be thinking like you are at great risk of losing something special, rare, or truly advanced.
For me, a great teacher has no rules about training elsewhere, as he/she requires no rules to keep students true to themselves and to the teachings. Sure, there may have been a time when technical information was a sensitive material, but that time is long gone now. Anyone that clings to that facade now, even for cultural reasons, is merely a person trapped by their ego and/or their attachment to their self-identity. This has to be especially true places that aren't even training daily in the first place. Rules, prohibitions such as these, like all departures from the Tao, if you will, tell you that disease is already present and that your best bet is to move on to healthier ground.
The question then isn't whether you will confront your teacher or whether you should be allowed to train at both places. The question is, "The Way is set before you. Will you walk on its path or will you follow those that have strayed from it? Will you follow the masses and seek your comfort and legitimacy in their numbers or will you seek comfort and legitimacy within yourself? Will you do what is easy or will you do what is right?"
Here's another angle:
Look and see how many students will stay right where they are, how in time the sensei's "rule" will be overlooked and ignored and how training will in time continue much as it always has. Now look at that number (in comparison to those that cannot or will not have training go on as it always has). Now look at the character and spiritual quality of these folks. What do you see? Do you see the strong, the one's capable of great integrity, or do you just see the masses, the way the masses always are - more marked by ease, fear, and convenience than by anything else? Before anyone talks about warriorhood, I think these things should be observed and reflected upon. In my experience, no "warrior" I ever wanted to support me when things turn life and death has ever been found in those masses that are more than willing to have training continue as if nothing happened, as if nothing was revealed.