Pro: the second line can be deactivated (“disconnected”) from the phone. Con: added cost.

Getting another number helps a lot in my opinion. If you're getting another sim in the same phone, I suppose it would be possible for whatever cell carrier is providing the service to be able to link your two sim cards to your identity. However, for other companies, I don't know of a way they could gather that your second sim phone number is linked to your first one. For instance, if you created two Discord accounts with your two phone numbers, you could theoretically be two completely different users that won't be linked together. I would personally go for a second number.

During covid, i made a Google voice number so i didn't have to give my real number when we were working from home. It was fine for that.

I got a second number for a while and saw no real benefits and went back to one

I have been doing precisely that. You have your personal number and the secondary number that doesn't have to be attached to your name if you want. Ofc law enforcement and telecom companies will be able to trace you based on cell tower locations and phone model.

Pro: An extra disposable number to give out whenever people ask you for "we need your phone number for ShittyDiscount". Whenever the spam calls get annoying I simply pop it out and place another prepaid sim.

Con: Phones with dual sim support can be rarer to find and be more expensive depending on your country.

Depends on your local laws and such, but in most European countries you can get a prepaid SIM card for a couple of euros/pounds/whatever at any supermarket, making them practically free. If you need a temporary number for a scammy special offer or any situation where your number is publicly visible (Gumtree, etc) it's a no-brainer IMHO.

If your phone suppprts running two SIMs at once, it has two IMEIs so as far as the network(s) are concerned it's two distinct handsets unless they deduce otherwise.

A fun aside: years ago I did some work for a small phone company (the company was small, not the phone) and they gave me a SIM with 100 numbers in a block and access to a portal I could manage them with. Sadly, I forgot to pay the annual £10 renewal fee.

This is a good question. Phone numbers are increasingly used as de-facto ID numbers, everywhere in the world. That's because, unlike email, they cost money, and in most jurisdictions you can't even get one anymore without presenting real ID. So: if you have a second phone number, you can effectively have a second persona for any site or app that requires phone-number ID. Seriously, at this rate, it's going to be all of them.

IMO the best use-case for this is to quarantine your contact list. That is, keep a separate number for social networks and messaging. The number you give to your in-person contacts will be instantly shared with all their cloud services, whether you like it or not. This is what allows Big Tech to triangulate and discover exactly who you know and therefore who you are. If the cloud services cannot trace a number back to any phone ID in their own books, then they can't do much with it and you will remain at least something of a mystery to them.