Two arms won't cut it. The platform will still have a degree of freedom to rotate out of parallel with the bed if you don't have a parallel arm setup - at least on one carriage.
Also, having a moving bed in Y negates the high-speed advantaged gained by having X on a "delta-hybrid" platfrom. I3's, for instance, are limited, in X/Y, by the Y bed moving, not the X-carriage.
You would definitely gain speed in Z but how much is that really an advantage? I don't find myself needing a faster Z-axis on my I3. What scenarios would you need a faster Z? highly unlevel and fast printing?
Also, having a moving bed in Y negates the high-speed advantaged gained by having X on a "delta-hybrid" platfrom. I3's, for instance, are limited, in X/Y, by the Y bed moving, not the X-carriage.
You would definitely gain speed in Z but how much is that really an advantage? I don't find myself needing a faster Z-axis on my I3. What scenarios would you need a faster Z? highly unlevel and fast printing?