Step 1: Pay Apple to do this, either by purchasing a Server model or by using the $ 800 mini BTO option. There is no step 2!
In case you want an alternative, you need to have enough cold blood to do the same as a user of the forums. MacRumors he ventured to try, but opening one of the new Macs mini and putting the second drive into gear is not even enough to change the RAM, in terms of ease.
Why would that be difficult? For two reasons: 1. this type of operation violates the mini warranty, requiring you to unscrew and dismantle several things, and 2. despite the space, a SATA connection cable is missing, as we well know from the teardown made by iFixit, you need to go after a compatible one and install it yourself. Below you see a $ 33 Bottom Hard Drive Flex Cable at MacPartsOnline, the component used to overcome this second limitation.
If these two obstacles are not too big for your desire to have a Mac mini with two drives, the process is relatively simple for those used to taking apart Ma products, but lay users should think twice (a thousand) times before doing something like this with a brand new device. On second thought, it might be a better idea to invest in a Mac mini Server.