[Joe Johnson]

There is nothing all that magical in this.

For motors with similar power ratings (e.g.250W BB + 340W CIM), the variations between matching stall torques or matching free speeds is not that big of a difference (~5:1 for stall torque, ~4:1 for free speeds). It is not that big of a deal, really.

For my tastes, in high power applications (which is why you want the extra motor isn't it?), I prefer to have the smaller sized (and in the case I discussed power as well) motor do less of the work (so that it will have to dissipate less heat which, because it is smaller is more critical -- heat transfer is a Area X DeltaT thing -- for the same heat to dissipate the less Area the more DeltaT)

In other cases, I only need the more power in one or other of the speed ranges. For drive trains, I think most folks add motors for more torque at low speeds. To that end, I tend to match stall torques. If I give up some speed on the high end, Que Sera, Sera...
Good luck.

Joe J.