Last I heard from friends of mine that have been deployed into combat zones, most soldiers carry a main weapon as well as a backup sidearm weapon for just in case reasons.
I doubt it is "most". Soldiers are not issued second weapons past a knife and many armies no longer issue those. Certainly soldiers pick stuff up, and they're not supposed to, but if they're the one carrying it and they're not slowing down the team, I suppose it is up to them. Point is however in SC, the masses in arms and armor, and the % speed you can move at depending upon how much you carry
were designed very deliberately to force you to choose not to carry ridiculous amounts of gear. If you use the calculator above and practice movement, you will indeed see serious differences at each step, and the slower you move the easier you are to hit. The stamina issue is even more important, and it seems to me you really don't want to be moving slower than 85% if you can help it.
Now if you really need a sniper rifle or a railgun, then you're the odd man out and you're gonna move more slowly; but seriously you do not want to do that if you can help it. Use the calculator and see the difference between 44 and 45 kg in your movement. It's huge. The stamina depletion issue is decidedly even bigger. Anyone can see this from watching the right vids on youtube, and virtually all the serious FPSers are carrying just one weapon.
BTW, choose wisely. I watched a vid where the guy loves shotguns and found himself completely unable to run a mission that way because even indoors the range is too short. What he found when he picked up the Demeco was that everything was massively easy because the Demeco kills things great indoors. That guy was carrying a second weapon, but he was moving slowly and was not a defensive player. Once death has consequences, he's going to have lots of bad habits to reform.
Also note on the calculator that only heavy torso allows you 4 grenades, and they are so useful that alone is reason to go heavy torso. You should also carry max med and oxy pens--2 each I think.
Printumus asked about the Multi-tool. That appears is a fantastically powerful thing to have on you! but it weighs 10 kg so you need to sacrifice armor and carry less ammo. They do this to force players to work in teams, or pay the consequences of terrible movement rates that will get you killed. If you're the sniper guy, the railgun guy or the "open-the-door-guy", you carry a light SMG and less ammo. If you're the point man, you're moving faster and if you're the sweeper you damn well better have an LMG.