How do multiple Mobile Processing Labs work?

Solution 1:

You can store additional data (analysed science) in additional labs but if they are connected to the same structure, the science must be different to what you've already analysed to be allowed to analyse it though (as opposed to having several labs disconnected from each other allowing the exact same science experiment to be used in each lab).

Data will not be processed faster per lab, but having two labs will allow twice the processing.

The rate is determined by how much data is in the lab, and the level and number of all scientists in the complex, not just in the labs.

Solution 2:

I have tried a multiple MPL setup, but KSP currently does not support using more than one MPL in one craft. When storing data it will only fill one of the labs, once that lab is full you will be unable to store data in other labs. This makes any other lab you have attached completely useless.

One trick to get around this would be to simply undock labs as they fill up with data. Once you're done transferring data you could redock the MPLs.