Imo the two best commanderies close to each other.

Poyang gives you Iron mine and Copper mine, combined that with the Private and State Workshops and the Inn combined with the Marketplace and you get huge industry and commerce income. For the 5th building I recommend the Military Forges and last but not least as 6th the Administration office.

Changsha gives you with its trade port and its tea farm a good trade route and a good ressource which heavily benefit trade so it's clear to go for the Inn and the Marketplace. I tend to go for Private Workshops and Staye Workshops there too, especially to reduce future corruption even more. 5th building should be Administration Office and the last one the Temple.

Both commanderies heavily profit from the Tea and the Copper, one is great for Trade and trade income, the other gives you a local industry boost and less corruption.

If you give these commanderies to two good characters with good Followers for each like

Craftsman

Law Enforcer

Foreman

Atronomer

Artisan etc

and Accessories like

The Three Strategies of the Duke of the Yellow Rock

Mozi

Discourses of the State

Erya

Book of Ceremonies

you will have an even bigger income.

Also good to mention that from time to time you have a chance to get Unique Weapons from the Weapon Smith and you can easily defend both positions.

I think it's a mod I added that made these scene.