Has it become banable to lvl alts with a twink with the help of multiple accounts?
Depends on your setup.
If you just got characters on follow and one character is doing the boosting, you won't get banned since its not against their terms of use, and you would win your appeal. When it becomes a bit of a grey area stroke dodgy is when you start to control multiple characters at once. Blizzard takes a huge disliking to automation and streamlining the process of multiboxing.
If you're looking to multiplebox and want to be able to control multiple characters at the same time then you need to take great precautions to ensure everything you're doing isn't against their terms of use.
Things to avoid:
Software: Using things like Hotkeys is one of the easiest ways of getting busted, yes it has a legit uses but when you go opening 5 WOW windows, Blizzard know, they also get notifications when windows become active, let me explain by this, when using windows, only one window is ever activated at one time, there is call commands within all software to know when a window is active, or inactive, when you have 5 WOW windows open, Blizzard know when you have 4 inactive, and 1 active, if using Hotkeys to active windows for example, You bind the keyboard numpad 1 to activate WOW1 and numpad1 and then bind key numpad2 to do WOW2, and numpad3 to do WOW3 and so forth, when you go spamming the numpad activating different Windows open, this is a redflag, Blizzard will see rapid windows are becoming inactive, active at greater than humanly possibly, while many people have appealed such bans and been successful with such setups, I have then see them to get banned again, and become unsuccessful.
If you're scrolling from Window to Window, and simply pressing one keyboard command at a time, its unlikely if you get banned, and if you do, its pretty safe to say you would win your appeal. Ask them to check the active windows, and key presses.
Hardware:
KVM
Using KVM switches that have a SYNC option to send commands to multiple computers at once is a HUGE NO NO! its very easy to detect and a fast easy way to get banned.
Programmable Keyboards with Macros
Using things like programmable keyboards to send a series of commands upon one button press, e.g number 1, sends key press 1, waits 2000ms and presses 2, key press 1, will result in a ban, in fact, its something you can get banned with 1 character let alone 2, 3, 4 or 5.
Natural Mindset
You need to have a "natural" mindset approach, my setup I use 4 NUC's all different brands/models, and each have their own HID (Keyboard/Mouse), I then use wireless numpads to control them, because you only have 2 hands, your limited to how fast you can do things, and everything looks more natural, varying delays between keypresses, the duration of the keypress, and the order of the spells is always different. I have never been banned but if I was to get banned, I would easily be able to appeal it and shown them that I am using 5 keyboard devices.
Programmable Keyboards
You can use programmable keyboards but great caution needs to be taken and you really need to know your stuff, I looked into this and yes it can be done, but you must do it right. You need to understand natural keypress deviation, what this means, when a key is pressed, each keypress is held for a different amount of time, for example you could write your name, lets pretend its "Andrew" The letter A will be held differently than letter N, and D, and so forth, there is an acceptable deviation before one character input becomes 2 letters because the key was held too long, this is processed by the operating system, and WOW also has this information, espeically more so if its a mechnical keyboard. The other automation is series of spells, this is by far the most serious offences and its extremely easy to detect, but extremely hard to avoid detection, you need varying key presses durations, and rotating spells, but too much randomness is also bad.
Hope this helps!