Rationale
Shadowlands will require a lot of regs. Since multiboxing has been hurt, the prices for regs will go through the roof. How to survive in this situation? Do it by yourself and get all the money/resources.
Method
Gather by two (five, eight) characters at once. You get 2..8 times more resources for the same time.
Guide
Step 1. Make a second subscription.
1. Go to your account subscriptions.
2. Click on "+create starter account" and make a new subscription.
Step 2. Configure WoW client.
Start the client using a newly created subscription (you can choose it in the launcher). Setup the following:
1. Make follow-macro with text
/follow Your-Main-Char-Name
Remember to replace Your-Main-Char-Name with its real in-game name.
This macro asks minion to follow you. You should use it before moving.
2. Make damage-macro with text
/cleartarget
/assist Your-Main-Char-Name
/cast Your-Main-Damage-Spell
Remember to replace Your-Main-Char-Name and Your-Main-Damage-Spell with their real ones.
This macro asks minion to damage your target. Use it if you occasionally aggred something.
This is enough to start multichar gathering. Additionaly you can enable Click-To-Move and install EMA addon - both make multiboxing life easier.
Step 3. Install the software.
Actually, you need no software at all. You can switch between WoW windows manually via Alt-Tab (or Alt-Esc). However, some software utilities may help.
Now (2020), since Blizzard forbidden input broadcasting, there are only two multiboxing tools without that broadcasting.
1. Wow Switcher (from the author of Levelbuddy) - minimalistic WoW windows switcher.
Pros: no setup, free for non-professional use.
Cons: only basic features, only for WoW (any version).
2. Joe Multiboxer (from the author of ISBoxer) - professional multiboxing software.
Pros: tons of features, good support.
Cons: paid, long initial setup, not ready yet.
Theoretically, you also can use scripting languages AutoHotKey or HotKeyNet, if you remove any input broadcasting from its scripts. However, it's difficult to make sure that scripts fully fit ToS so this way is not recommended.
Step 4. Enjoy your new experience!
Try multiboxing for free by using low-level WoW characters, make sure it works, and then use it for fun and profit in Shadowlands!