Still takes the same amount of energy to spin the engine over regardless, but with higher voltage you have more pressure pushing the amps through the circuit, and you can get away with smaller gauge cables.
Yep the only advantage to a 24V system is that you can use lighter cables. It still takes the same amount of energy to spin your starter. A lot of boats run 24V due to the longer wire runs.
With the 30" cable runs run to the starter in an IDI, this is a solution in search of a problem. If your batteries were in the back of the bed i'd say its something worth considering. Yes you only need half the amps at double the voltage, but by running your batteries in series you have doubled the voltage but cut the amps available in half.
Ex. I have two 12v batteries capable of 800 CCA each. Wired in parallel as they are, they can produce 1600 amps for cranking at 12v. With heavy gauge cables and short runs to the starter there is very little loss.
You wire these same batteries in series, you have 24v but you can only produce 800 amps now. Unless you have a loss problem due to a long cable run or a restriction on the gauge of cable you can use, there is no advantage to be had over 12v.
In my opinion, you are much better served buying a high quality 12v gear reduction starter that lots of IDI owners have proven reliable, than by trying to whip up a 24v system just for the hell of it.