It is most effective if you have 30 drones at one field, but could you get more minerals, if you build two bases there, and collect from both at the same field?
First of all, if you have 2 bases near a mineral patch, your drones/svcs/probes will always go to the closer base (rendering the other base obsolete).
The optimal amount is much less than 30 drones (30 is the amount where it is not possible to harvest minerals any faster from the given mineral field).
In HoTS, a fraction is displayed above your command centre/hatchery/nexus indicating how many harvesters you have over a suggested maximum of 24 (for bases with 8 mineral patches).
Refer to this page on liquipedia, which has an interesting graph about optimal amount of gatherers.
So there are two issues with this proposition.
The first is a practical one - there are no maps in the standard map pool (or campaign maps) that one could conceivably build two command structures (Nexus / CC / Hatchery) in an efficient manner.
The vast majority of the time, there is simply no area to build on the far side of the mineral and gas line (hindered even further since the resources often concave in a circle towards the ideal command structure spot).
As TylerAndFriends mentions, workers will always go to the closest base structure, which prevents the 2nd structure from providing any advantage.
But let's assume that there is some hypothetical map in the future exists where you can build two command structure's equidistant from one or more mineral patches (a straight line of minerals, buildable on either side is the simplest example). So would those two bases allow an increase in the amount of harvestables?
The answer is no, because you run into the second problem -- harvesting is per worker, not per base.
Harvesting in Starcraft 2 does not care where the harvesters are coming from; only one harvester is allowed to be harvesting a mineral at any given time, with other workers forming an orderly queue behind them. So having a 2nd base (which would conceivably let you send more workers to harvest), would not increase your efficiency, because past a certain point (3 or so), increasing workers / patch does not increase efficiency.
So, due to both of these issues, it is never economically viable to attempt to harvest an expansion with multiple command structures.