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

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.