I went with both (mainly because I like different usernames for different apps/sites) but see what looks like people using one login for both.
Mlem appears to allow that
I’m using Mlem so of course this may not be relevant but I had kind of the same problem. I expected to be able to post from my feeds page but you can’t. You have to be inside the actual feed you want to post to.
The tongue was the main reason I wanted to use Mlem!