Could you elaborate? What makes the secondary one more secure than the first one?
Nobody knows about it. Sim swappers will attack your number. The one they know.
They may be able to bribe phone company employees to look you up by name, but you'd at least be blocking one route of compromise
Which company? :)
All of them, probably
Something prepaid.
You can get phone service anonymously in the USA.
Having a separate number/SIM solely for 2fa makes it harder for attackers to know the number they need to initiate a simswap in the first place, since it won't be publicly associated with you the same way your normal phone number is, like being listed on your LinkedIn or resume, or obtained from a social media or other account info leak, or able to be socially engineered or phished out of friends/family.