As with many modules in a larger system "One is good but two is better". For Frequency Shifters the Dual FS 65B is a two for one and includes Ring Modulation and phase effects modes as well.
The 65A is an entirely digital interpretation of the standard Bode Frequency shifter model which achieves numerous significant benefits over its analogue predecessor, primarily the dual configuration.
Other advantages include stable low distortion quadrature sine carrier for accurate sideband generation, digital math eliminates feed thru error found in typical analogue multipliers . No need for squelch circuitry. Other benefits include absolute stability at low shifts, low noise signal path and zero maintenance calibration ...EVER. There are probably only a handful of Bode FSs in the hands of users that are ever within factory specs at any given time due to complicated and costly calibration procedure. With the 65A that will never be an issue and you will always have an accurate and precise instrument regardless of ambient temperature or age. Analogue Frequency shifters still maintain superior high frequency response over digital but due to the nature of the effect the difference is small.
Another important consideration is that it would be both cost and space prohibitive to implement a matched set of dual FS in analogue with similar features and performance.
The real fun begins when the two independent FS sections are internally patched in series and parallel configurations via the mode selection control. This allows for some seriously altered stereo effects with near 3D results.