Yes, as long as both batteries have the same Voltage and Amp Hours you will be fine to wire them in PARALLEL (positive to positive and negative to negative. You will then have a 48V 24Ah battery which will get you twice as far.
You don't ever want to mix batteries with different Amp Hours or different Voltages.
Where are you getting this 500W and 750W figure from? It sounds like the rating of the motor rather than the battery. The rating of the motor doesn't matter in this case. The battery will still power a 500W or 750W motor regardless.
