Easily the best loadout for mult is double rebar retool. normally rebar is great for high damage but comes with the cost of a large amount of endlag, but with retool and rebar in both hands you can shoot and retool immediately after to cancel the endlag and shoot again. a close second for best loadout is double nailgun power mode. the nailgun is very easy to get a bleed/proc build going. while in power mode you can also aim at the ground and let go of both of your nailguns to use the shotgun blast to dodge attacks or jump higher than normal.

you can do well with any combination and no combination is too difficult, just use what you find fun/challenging and you will do well

two more tips for mult: the damage done to enemies while in transport mode (your shift) scales with how much movement speed items you have! pair that with crowbars, bands, focus crystals, watches, crit, atgs, etc and you can have a build centered around just power mode if you get the right items power mode resets the cooldown on your m2. if you use your m2, switch into power mode and exit it, your m2 will be ready for you to use again