I tried duoing with some one who is D4 and I am P3 and it said the difference was too high? I swore I thought it was a whole rank above and below which was the cap.
It's different across the ladder. Tighter ranges higher up. Iron can use bronze and silver.
Edit. Aight... Apprently that's not good enough so heres the proof of what I said.
"Iron Tier: Can only queue with Iron, Bronze, and Silver Tier players. Bronze Tier: Can only queue with Iron, Bronze and Silver Tier players. Silver Tier: Can only queue with Iron, Bronze, Silver, and Gold Tier players. Gold Tier: Can only queue with Silver, Gold, and Platinum Tier players. Platinum Tier: Can only queue with Gold players, Platinum players, or Diamond players who are at most two tiers above them (so Plat I can queue with Diamond IV and Diamond III, Plat II can queue with Plat I and Diamond IV, but Plat III can only queue up to Plat I). Diamond: Can queue 2 divisions up or down (so Diamond IV can queue with Plat I, II, and Diamond II, III, etc.). Master Tier: Can only queue with Master, Grandmaster, and Diamond I and II Tier players. Grandmaster Tier: Can only queue with Diamond I, Master, Grandmaster, and Challenger players. Challenger Tier: Can only queue with Master, Grandmaster and Challenger Tier players"
Sourcehttps://support-leagueoflegends.riotgames.com/hc/en-us/articles/204010760
Diamond+ can only duo with 2 division up or down, so d4 can duo from p2 to d2. https://support-leagueoflegends.riotgames.com/hc/en-us/articles/204010760#h1q3
