As atomic_wizard mentioned above, as long as you follow that guideline and make sure you don't use the account for personal gain on your main account you should be good to go.
Basically just treat your alt account as if it was your main and only account, and make sure to not connect when your main account is banned or alt for that matter.
I used an alt account, although nobody noticed or cared, and it was fine, I got to explore the ways of a new player over again, of course, easier since I got the hang of things long ago.
I no longer have access to said alternate account but was fun while it lasted.
The alternate accounts getting banned are just those who try to connect with them while banned on main accounts.