Missions & Complexes

I'd probably go with dual Domi's and run missions with drones. Or maybe ishtars. Sick drones on rats, and then loot salvage while running mission, while other box just afk's. Just aggro entire room, have one domi follow the other around. Make sure you have a cap stable tank, that can tank the entire room.

6 toons doing PI.
For missions - One in a BS and the other in a Noctis - Send the noctis (i call this my cleanup crew) after missions is clear or ending, then move the BS on to the next mission.

Meh, this is what I do, however, I'm sure I could be more productive or more efficient.

One to shoot sleepers, one to salvage their wrecks.

I don't mission, hate the grind.

I use intel gained from knowing the area I live in and experience and then take opportunity depending on what's availble.
This normally means running plex's and if not ratting or hunting for Officer/Faction spawns.

Having a logistcal alt helps a great deal for helping you tank tougher plexs, moving your stuff from A to B, warding off enemies while you rat, helping you clear down belts faster if the area is secure.

Sure the money intake is random but the good days make it all worth it.
Probably not advisable if you want an expected income monthly but I've never made better ISK or had more fun while doing it.
Got to love finding highend loot in hostile environments.

And it doubles your traveling time, so sending both toons to different missions halfs your travel time, and only increase your mission time for local rep. And local rep is only important if your dps is to low. In which case your skills are low, and going in with 2 ships, one with tanks and one pure dps will anyway help you. Like a drake and a high dps bc.

Learn to run two separate missions simulataneously on each character. Once you get hang of it this beats having two characters in one mission, isk/hr wise.

My personal method.

Find a system that has 2 level IV agents in the same station.

I have me in my Nightmare/Tengu/Nighthawk/Astarte(considering changing this up for a Talos) (mission dependant).
I have my alt in a very, very heavily tanked Dominx with Sentries, Heavies and remote Rep drones (shield/Armor dependant) with Remote armor or shield reps (mission/ship dependant on the above).

I then accept a total of 4 level IV missions.

I will blast through them... fast. A combination of high DPS main ship with a logistical Dominix with sentries makes for quick missioning. I rarely worry about aggro because of having all that tanking locally and remotely.

Once I complete those 4 missions, I will run back. Hand in all 4, then send out the alt, to salvage & loot all 4 missions.

Whilst I am doing this, I am running 2 level 4's with me at the same time. Usually, this will end at the same time as the salvaging, at which point I will make a judgement call on whether to loot & salvage those two, or just begin the cycle again.

I could just do the blitz of 4x4x4x4 all the way through, without looting, but I have a little manufacturing plan in mind for the future which I would like to try out...

As others have said, there is better and quicker ways to make money, but this works for me and keeps me engrossed. I like trying to multi task a lot, so... its fun.

My personal aim is to move onto a WH at some stage, as despite what I am doing being quite good, I want something new. The general opinion is a multipurpose dual box will make more money in a wormhole, from what I have read.

Take it easy.

if youre talking about high end multiboxing, you can run full capital class 5-6 WH escalations with 6 accounts.
that will earn you roughly 2bil per hour.

obviously it requires high skill level characters and a good ability to multibox.
you also need to know how to do the sites efficiently but that's another topic.

Meh, I do missions on Neesha and have a alt to Noctis-salvage, scrapmetal-process mods and haul stuff around for my main account (which isnt neesha but doesnt have to be included in this to stay on topic).
The salvage/scrapper alt also serves as my cloaky scout and marketeer/contract hunter.

Can even drop him into some PI, if i can be bothered.