How to hire companions in Skyrim

You can use a Calm spell on them to stop them fighting. If you don't have Calm (or a staff or scroll or weapon enchanted with calm) you're going to have to fast travel (if you can) to somewhere else, or enter a building etc. Anything to make you go through a loading screen. When you come out on the other side, your two companions should be pals again.

(They shouldn't be able to kill each other, so maybe letting them fight it out will cause them to reset when one of them kneels down and yields.)

A sneak build cuts though 95% of the game like butter, but it requires some adjustment for the other 5%: dragons, surprise outdoor encounters, bosses that take you out of stealth for a dialogue when you enter their rooms, and some melee bosses with high armor and health and a location that makes it hard to sneak away in between attacks.

1. Pick up a horse and a heavy armor follower, and consider putting points into conjuration, to provide lots of tankage when you need it. Make sure to keep your follower tank in good gear (in the latest version they now level up along with you.) In dungeons you'll want to leave your follower behind and go get them when you've cleared up to a boss, because the AI stinks for stealth support.

2. Invest in a bit of stamina, wear light armor, and kite like crazy (sprint away and then shoot, using the unrelenting force shout to help keep enemies at a distance). This gets much stronger once you pick up stagger in the archery tree and max out unrelenting force.

3. Get the 3x bow and 15x 1h damage perks in the stealth tree as soon as possible.

4. Put points into smithing and stay at least a bit ahead of the gear curve, and do at least a little enchanting (+fire/frost weapon enchants in particular seem very strong out of the gate) and alchemy to stock up on poisons for your arrows. Because of this, right now I'm stealth one-shotting everything in dungeons but bosses. This lets me save up potions and scrolls for those few tough encounters.

5. Look into going down the illusion path for muffle and invisibility. I haven't done this personally since you don't want to spread yourself too thin, it doesn't help with dragons like conjuration, and I like the occasional big messy surprise fight, but I hear it works well also.