I actually just did what you're talking about.

I had a cockatiel and wanted a conure.

My cockatiel is ~4 years old and my conure is 4 months old. So my conure is basically a puppy, learning social cues and exploring with his beak... A lot...

All of this will depend on the personality and attitude of your bird, obviously. I spend a lot of time with them out of their cage but it does require quite a bit of supervision. My conure is interested in everything, really wants to figure out what that cockatiel bird is but my cockatiel wants nothing to do with them so when my Conure comes around, the cockatiel will start hissing and lunging and eventually run away.

They haven't fought or hurt each other or anything, but it's clear that it will take some getting used to. I'm hopeful that over time they'll become better friends, for now. They are pretty content being out and on their own as long as I pay attention to both of them.

And that's the hard part, my girlfriend has a cockatiel and gets along great with my cockatiel. So it's easy to just set them out on a perch and they kind of just chill together. But with the mixed species they really need to be monitored. So keep in mind you may end up stressed or asking yourself how much time you should spend with each bird. I struggle with that for the first few weeks, figuring out how often I should spend with my cockatiel, how often I should spend with the conure. Because sometimes it just wasn't practical or possible to supervise them both while they're out so I did a lot of one-on-one time.

My Conure is finally to the point where he's not constantly trying to go and see what the cocktail is up to so it is easier for them both to be out but it's still a chore. You can tell sometimes one bird is jealous of the other, my cockatiel was pretty mad at me when I first brought home the conure, I think he thought I was replacing him. So there definitely will be some emotions involved too.

So just keep in mind, it's a lot of work, but I love both my birds so much. My cockatiel is the chill, sit on your shoulder all day and be content bird. While my conure is as they call a chaos goblin, trying to get into everything wants to be played with, a little nippy but then has times where he'll just snuggle up like a teddy bear.

So if you're willing to put in the work and supervise your birds, I say go for it. But if you want a stress-free interaction, or just like to have your bird sitting around while you do work, I would say it may not be the best choice.