You are making the problem too hard. This is a simple Ohm's Law application. The current at any location will be the voltage at the source divided by the total impedance along the path from the source, to the fault point, and back to the source. So if you start with two identical utility connections, and go through two separate transformers that have identical impedances by different turns rations, and from there you have identical secondary feeder sizes and lengths and identical panels and identical everything else, so that the impedance seen from the point of view of the source looking out at the system, then the fault current will be higher on the system that starts with the higher voltage. 480 divided by a value of impedance will be higher than 240 divided by that same impedance. Ohm's Law shall prevail.
But in a practical sense, if you have different secondary voltages on two transformers, then the stuff downstream of the two will not be identical. The higher voltage transformer will have a lower value of 100% rated current, so its secondary conductors will be smaller. That means they will have a higher impedance for the same length. Therefore, if you postulate a fault at the same distance from two transformers, one at 480 volts and the other at 240 volts, you will get one fault current of 480 divided by an impedance value, and another fault current of 240 divided by a lower impedance value. Which one has the larger result? Without knowing the exact values of secondary conductor sizes, the answer cannot be guessed.