For clarity, you are saying that I (as a sole owner of a singlesig wallet) could create a multisig 2 of 3 set up by adding two more wallets while continuing to use my original singlesig as one of the signers? I did not think that was possible. My understanding was that a multisig set up like this needed to start fresh with all new keys so, that’s how I set mine up.
Discussion
A 2-3 multisig needs 3 seeds (private keys), you can reuse the seed you used for singlesig, as one of the 3 seeds.
Your funds are seperate from eachother, because your singlesig generates a different public key (and thus addresses) than your multisig.
As an example, when you have a 2-3, with 3 seeds, A, B, C... You could switch one seed for another: A, B, D.
This now gives a totally different public key and addresses, so basically a different wallet, while A and B are reused. Note that this is just an example, not that it's smart to do this last example 😂
Playing around with Sparrow wallet helped me a lot to make it all click in my head. Also this tutorial by nostr:nprofile1qqs27tmg9agj3xv993n9flcxt2jqtfqazh3trjmvj9e3yds9w38sdespzemhxue69uhky6t5vdhkjmn9wgh8xmmrd9skcqg4waehxw309ajkgetw9ehx7um5wghxcctwvshusv9d
Again, I don't know the implementation in BTCPay Server, which might give you a different interface 🙂
Nice nostr:nprofile1qqspnzgrfett3asxcuj0gksje6z2zxzpvgd27uvz58m9vsuqh8zzw6cpz3mhxue69uhhyetvv9ujuerpd46hxtnfduq3jamn8ghj7cn4vakkzm3wd4nh27fwdejhgw358q6rsj0csxp recently posted a very long Sparrow tutorial, as well, which is taking time to work through.