The latest generation mobile phones demand for new figures of merit to evaluate the potential of the antenna system since the earliest design stages. We present a novel method to evaluate the isolation potential of antennas that operate in different frequency bands, generalizing the concept of electromagnetic isolation. Simulations results show that as the antenna bandwidth widens the electromagnetic isolation gets worse. In particular, the duplex isolation improves very fast as the antenna bandwidth shrinks. This suggests that using different antennas for the uplink and the downlink might be inherently a promising alternative to conventional antenna paradigms.