AxisManager concatenates on two axes weirdly

[mhasself]

When concatenating AxisManagers that have two shared axes with disjoint elements, the concatenation works properly on one axis and truncates the other.

This sort of concatenation should either concatenate both axes properly, or fail with information

>>> from sotodlib import core
>>> a1 = core.LabelAxis('a', ['a1'])
>>> a2 = core.LabelAxis('a', ['a2'])
>>> b1 = core.LabelAxis('b', ['b1'])
>>> b2 = core.LabelAxis('b', ['b2'])
>>> X1 = core.AxisManager(a1, b1)
>>> X2 = core.AxisManager(a2, b2)
>>> core.AxisManager.concatenate((X1, X2))
AxisManager(a:LabelAxis(2), b:LabelAxis(1))

It would be great to see b:LabelAxis(2) there!