DUO QUEUEING
DO I HAVE TO DUO QUEUE WITH SUMMONERS FROM MY LEAGUE?
No.
HOW ARE DUO QUEUES MATCHED IN GAMES?
You will be matched against opponents based on your average MMR. In champion select, if either player is chosen as first pick, the team captain will be assigned first pick and the other member of the duo will get whatever the position the captain originally received at the start of the match.
WHAT IS THE PENALTY FOR QUEUE DODGING IN RANKED?
Detailed descriptions of queue dodging penalties are available here. This link also includes information about missing Ready Checks.
WHAT ARE THE RANKED DUO QUEUE RESTRICTIONS?
We have introduced a new restriction for ranked duo queues as of patch 4.11. Players can only duo queue with teammates ranked within one full tier above or below them. For example, a Silver 3 player may duo with players ranked anywhere from Bronze 5 to Gold 1.
If you were ranked in solo queue during the previous season, that rank will be used to determine your duo eligibility during your placement matches.
If you were unranked (or never played ranked games before) during the previous season, you will be allowed to duo with Gold, Silver, and Bronze players while your skill level is being assessed. After you have been placed, the duo queue restriction will apply based on the tier you have earned in your placement matches.
https://support.riotgames.com/hc/en-us/articles/204010760-Ranked-Play-FAQ#h4q2