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In this paper, we study the utility of the Common Mode (CM) signal for Far End Crosstalk (FEXT) cancellation in upstream Very high-speed Digital Subscriber Line (VDSL). We begin by extending the stochastic channel model developed for Differential Mode (DM) signals to model the channels of both CM and DM signals. The characteristics of the resulting CM-DM joint channel are first studied. This is expected to help in the development of computationally efficient algorithms for FEXT cancellation. In, the capacity improvement with use of CM-DM was studied for the overall channel. The rate gain with CM-DM joint processing as compared to DM only processing is shown to be a strong function of frequency and the loop length, an observation that is of critical importance for development of FEXT cancellation algorithms. It is shown that such joint processing is beneficial for longer loops and for high frequency subcarriers.