From personal experience I've never used onetap on my main and it hasn't been banned. Make sure your accounts aren't linked together with the same phone number or if you have family share on make sure you dont family share from your main to the account you will be cheating on. As long as you dont open onetap and login to your main account you should be fine as even if onetap gets VAC detected as you havent used it on your main it wont get banned, unless you get an overwatch ban
This is my knowledge on this.
For anyone else who sees this correct me if im wrong