What are the pros/cons for duo queueing in ranked

For the most part if you Duo you will go up more (if you have skill to climb)

Where SoloQ fails is that you have 4 chances to have a idiot on your team.

With Duo you only have 3 (unless one of you is the Idiot) and even then if you two work well together you can counter 1 idiot on your team or abuse one on the enemy's team.

The pro is that you know the person, can potentially have coordinated plays with them, and if they have a higher mmr than you than you'd gain more LP for wins than if you solo'd, and lose less for losses.

When you're in a premade, you're basically matchmade based on the average of your MMRs. Meanwhile, your LP gains/losses depend on your MMR relative to the division you're in, and your MMR gains/losses depend on your MMR relative to those in your match. As such, if you're the weaker player in the premade, you'll have a lower MMR than those in the match, and stand to gain more MMR for your wins, and lose less for your losses, and that quickly translates to overall increased LP gains if you win at least 50% of the time. (Of course, you are theoretically playing against harder opponents than you're used too though, so unless you actually deserve a higher mmr then you may have a lower winrate than you're used to.)

The reverse is true for the higher MMR'd member of the duo, but there the idea is that since they're playing against theoeretically weaker opponents and have the added comms of their duo buddy, they'd win more often which would negate the smaller than expected MMR/LP gains.

If you have basically the same MMR, then it's just the added benefit of having someone you're reasonably confident you can rely on to play a certain way, instead of the crapshoot that is a random player you're matchmade with. That said, you're more likely to be matched against other duos when duoing yourself, so you'd have to decide for yourself if it's worth it, as while you may have more coordinated plays, so will they. (Your LP/MMR gains/losses would be the same as if solo provided you got matched against another duo.)

In theory, the pros/cons should cancel out to make no net difference compared to true solo, but in practice it'll really depend on your synergy with your duo partner (smurfs aside).