This is a bit of a topic drift, but anyway:
My takeaway from the above is
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Towers are capacity constrained, and adding an additional radio (e.g., a second SIM card to an HD2) on the same frequency band changes the per-radio capacity available at any one particular point in time. The capacity constraint may be the result of frequency saturation (many users for a given band), backhaul capacity constraints and possibly other limitations. The approximate effect of adding an additional radio is then to reduce the per-radio available bandwidth from C/N to C/(N+1) where C is the total capacity of the tower on the particular band and N is the number of radios requiring access. If the tower is designed for only a few users then the reduction may be significant.
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Strategies for avoiding this effect in some cases are then essentially to ensure that the different radios are using different frequency bands, e.g. by employing more than one carrier, or by using the band selection option to distinguish the two radios within a single carrier deployment. If the backhaul is capacity-limited then there is precious little one can do, except ensure that the different radios connect to different towers (i.e., different carriers, at least in some cases (carriers do occasionally share towers and the backhaul)).
Using more than one band (or carrier) can itself be dicey, since some bands/carriers are better/have greater capacity than others (depending on the location) resulting in low bandwidth for the unlucky radio.
Fair summary?