Sorry friend, this is the case. There must only be one slot.
Interesting note re: RAM choices ... if you select the i5-5200U CPU on the Lenovo US website then you are allocated 4GB of RAM and have no option to select 8GB. The mid-range CPU (i5-5300U) allows you to select either 4GB or 8GB. Finally, if you select the i7-5600U then you are allocated 8GB and can't select 4GB.
I wonder if it's technically useful or if it's a commercial move. I have an Ideapad Y470 (a model from 3 years ago), and I got the impression they tried to limit the RAM below what the hardware could run, maybe because they want to segment the market in a certain way.
The customization page on their website only allowed 8GB of memory and the user manual explicitly stated that the maximum RAM was 8GB. I bought 16GB after reading users reporting that it worked fine, and I can confirm: it works fine. I push it to the limit all the time.
Then again, it might just be that new RAM chips came out after the laptop did, and I might just be conspiracy-theorizing.
This is the logic board on the 2014 15" rMBP: https://d3nevzfk7ii3be.cloudfront.net/igi/PmBmFGBkmnHDvEpo.h.... It's wedged between the fans and the batteries, and there's memory on both sides. If you make the board larger to put in SO-DIMM slots, you lose either battery or cooling capacity. The rMBP manages to fit a 15.4" screen and a 95Whr battery in 4.5 pounds. I've never seen an upgradable laptop that fits that much battery into that weight class.
Not that I've had the chance to try it out but the concept seems great, especially for people aged 50+ IMO.
http://store.sony.com/digital-paper-system-zid27-DPTS1/cat-2... [Sony]