Sorry, but your graphic is just not conveying your situation very clearly. I am not certain I understand what is connected to what. But I'll take a stab at it.
In both situations, you appear to have a main panel that has its own main breaker. It also has a feeder breaker that provides power to two other, separate buildings. I get that part.
In situation A, you have an MLO panel with 6 breakers. That is OK. But how is power getting to the third panel? Does it come from the main panel, on the same feeder breaker? If so, then I think that is OK. Does it come from one of the 6 breakers on the second panel? If so, then I think that is also OK. In either case, it seems to me that situation A is OK.
In situation B, it seems that power to the third panel is tapped from the feeder that serves the second panel. Now, if I understand that the feeders are all the same size, then this is also OK. It is not a tap, as you correctly pointed out. On the other hand, if the feeder to the third panel has an ampacity that is less than the rating of the upstream feeder breaker, then you are into the tap rules. In that case, that feeder would have to terminate in a single breaker.
Charles E. Beck, P.E., Seattle
Comments based on 2008 NEC unless otherwise noted.