It's one pattern vs another. It's true they fight characters with similar ranks within an organization, but it's equally true Luffy's opponent from the same arc should be tiers above theirs. So the theme doesn't really fit. Unless Imu or someone on that level is the main villain, Sanji and Zoro won't fight admirals in the same arc.
As for the issue of Luffy's oponnent not being tiers above theirs. That isn't that big of a deal when Luffy vs Akainu is a grudge match. A personal beef Luffy needs to take care off. Oda can bend the rules for this occasion, it's not that it makes a massive difference. Akainu established his superiority in strength to the Admiral rank in canon.
I fail to see why that disqualifies the entire fight itself. Honestly to me it looks like splitting up the Admirals fight only serves to save Zoro from the power scaling implications that fight would have compared to Sanji.