You can just think of that other account of yours as a backup. Think of it this way: your account lives in a server (kbin.social, lemmy.world, lemmy.ml etc). When you make another account on another server (i.e., you make an account on lemmy.world), that account lives there in that separate server. So you can have two Brad_Braces (even ten if you feel like it) but of different servers. There’s no conflict anyway
You can access and comment in Lemmy from kbin, but I’m not sure if you can do the same for Mastodon. But Mastodon users can certainly use their accounts to comment on Lemmy. The beauty of the fediverse (or threadiverse, as some guys like to call it)