if weight is the only parameter, then i think that you should go for 2 batteries instead. One 12v 12Ah lead acid (that i see the specs right now) weights about 3.9Kg, while one 12V 7Ah weights 2.7. So you may wanna use 2x 7Ah 12V in series and get 24V 7Ah for 5.4Kg, 600 grams more than the 12Ah 12V. I think his is a fair trade, plus you save the extra cost for the circuit. Not to mention that you will be needing a powerful transformer for your dc-dc converter which means extra weight. Plus, the efficiency of 2 batteries in series is much better than using a DC-DC converter.

And something more about efficiency. Power loses due to heat are a result of the current and not the voltage:

P=I^{2}xR

Which means that, if you need to draw 48watts for the motor, if the power source is 24V, the current will be I=P/V = 48/24 = 2A

But if the power source is 12, the current will be 48/12 = 4A

So, the heat losses on the wires for 12V battery are much higher than 24V.