I think you will find this hard, mainly because soldiers are supposed to look the same. you mentioned another character but didn't show them, so I'll try my best based on what I can see. the questions I would ask myself are. are they the same rank, regiment, army? are they both foot soldiers? Do they have the same duties and operations? if its yes to all of that then that means you want the same silhouette, and the difference is in the character. is one of them strict, laid back, older, younger, bigger, smaller, well kept, rough and dirty. if one of them is more scrappy and against the rules, they could wear the belt and accessories across the shoulder. one side of the collar can be bent down. a the sleaves can be loose or rolled up. Lastly, things like patches, broaches, pins, badges, dents, bands. personal items, like binoculars, extra bags, hip pouches, radio. if the regiment is less strict than personal weapons, like knives, pistols ammunition. Unless your making a whole comic, then just do two version of each character, them in neat solider uniform looking the same, with badges, patches or personal item to distinguish them. and them in down time, relaxed where their personality can come through, in rolled up sleaves, open collar, wonky helmet. and draw them side by side, making one smaller always helps. the more cartoony you go the easier it is to make the difference in physical proportions.