half-circuit analysis techniques

We use half-circuit analysis techniques to simplify the analysis of symmetrical differential amplifier circuits. Consider now the implications of using a half-circuit to estimate the high-frequency breakpoint of a differential amplifier.
(a) How would the om of a fully symmetrical differential amplifier be related to the ?HI you would calculate for the difference mode half-circuit of this amplifier?
(b) Clearly you would estimate a different ?HI if you used a common mode rather than a difference mode half-circuit. What does this mean?