Role of Electrolytic Capacitors and whether they can be replaced by Film capacitors in battery operated power converters
As others have stated, both specifications are important, and there are other reasons for wanting capacitance on the power lines: preventing voltage droop from the battery if it can't supply current quickly (battery chemistry impacts this significantly), I*R losses if the battery is far away, path for motor transients (though I'd expect diodes and smaller ceramic caps, not electrolytics, across the bridge transistors for the faster transients).
This is questionable; you've left out tolerance.  If the dual caps each have +/-20% tolerance, and you get one at +20% and one at -20%, they're not going to share current evenly because they have different impedances, which could lead to one failing prematurely, and the other shortly afterwards since it may not be able to handle the load by itself.  Also keep in mind that higher tolerances tend to cluster near the outer limits, because values closer to nominal are typically sold with tighter tolerance specs for more money; that is, a +/-20% part is not likely to be within +/-10% tolerance, because those will be the +/-10% parts, etc.
The reduction in capacitance is not recommended.  If 15 uF were sufficient for the intended load, the original designer likely would have gone with a 15 uF electrolytic instead of a 1500uF electrolytic; for the same voltage and current ratings, the 15 uF would be substantially cheaper and smaller.  If you're using a different kind of motor than the original designer had in mind, you'd need to look into the suitability of the lower capacitance yourself.


If you're really dead set on getting rid of that particular capacitor, I'd  look at increasing the PWM frequency (so you can then reduce the bulk capacitance required), if that will work with the motor you're using (the motor will have its own mechanical and electrical time constants that your circuit has to deal with).

Another thing to consider is the impact of motor inductance that's in parallel with this capacitance.  The higher ESR of an electrolytic can sometimes be beneficial in dampening potential LC oscillations that might otherwise be possible as a result of pulsing power to the motor with the bridge, particularly if the motor winding resistance is low.  You'd need to look into the impedance of the motors you want to drive to check for potential resonance.  

As a general note for future reference, Analog Devices (no affiliation with myself at time of this posting) has an article I've found useful on the general parasitic effects of capacitors in their "Ask The Applications Engineer # 21 : CAPACITANCE AND CAPACITORS"