If you took the bottom cover off, then you can see the easy-to-access memory bay already. This is the one that has paired slots #2 and #4, and they are completely visible. If you can see the one 8GB factory memory card from Lenovo installed in one of those two slots, then your second 8GB card (no matter what brand or who you buy it from) goes into the other of these two slots. If both of these completely visible memory slots are currently blank, then you obviously have the 8GB factory memory card from Lenovo installed in one of the two paired slots #1 and #3 under the keyboard.
So you yourself can see for yourself right now whether you're going to have to remove the keyboard or not in order to install your after-market second 8GB in the other slot of the pair in which the 8GB factory Lenovo memory card is already installed, so as to achieve dual-channel mode or not.
The terminology of "memory bank" or "memory slot" or whatever you want to call it, don't be distracted by terminology. Just look for yourself, and you'll have your own answer. WMIC appears to reference the memory slots as "banks", but these don't seem to be numbered the same way as other programs (such as highly recommended CPUZ which you should also have installed and available for your use) identify them.
For example, on the desktop PC I'm currently working at I have 2x4GB of memory installed in two of the four DIMM slots (let's just call them slots #1, #2, #3 and #4). In particular, the two memory cards are in slots #2 and #4. When I run CPUZ and look at the SPD tab (which shows exactly what memory is installed in each DIMM slot) it tells me I have 4GB in "slot #2" and "slot #4". But when I run WMIC it tells me I have 4GB in "bank 1" and "bank 3". Obviously the two programs simply have different terminology, with WMIC referring to "bank 0", "bank 1", "bank 2" and "bank 3". Clearly it is the same two physical memory slots that hold each 4GB memory card, and it's just program terminology that differs in describing the two occupied slots vs. the two vacant slots.
So, back to your story. My guess is that your discovery through WMIC that you currently have 8GB in "bank 2" (out of bank 0, bank 1, bank 2 and bank 3 which is how WMIC describes a 4-slot machine). I would predict if you used CPUZ you would see that this 8GB is installed in "slot #3" (one of the two under-the-keyboard paired slots #1 and #3), just as I have said Lenovo does with their factory memory. If you bought two memory sticks from them they would be in these slots #1 and #3 under the keyboard, and then you would be able to easily install your own additional memory in the easy-to-access two other paired memory slots (#2 and #4) on the underside of the laptop under the bottom cover.
So, use your eyes and see if the open two slots in the underside memory bay under the bottom cover are both empty or not. The rest is obvious. Also, if you use CPUZ I'll bet that Lenovo memory you say WMIC identifies as being in bank 2, is in "slot #3" according to CPUZ... i.e. under the keyboard.