2X NVIDIA GEFORCE GT 755M SLI + 8GB RAM or 1 NVIDIA GEFORCE GTX 850M + 12GB RAM?

Sorry, I don't get quite the question. I guessing mean which one is better?

The one with GTX 850M is good but
The one with 2X GT 755M + 8GB RAM is brillant probably the best

I worked for IBM and then Lenovo for 32 years in Procurement Engineering and know all of the parts by performance. In here, I also learned relating to gaming tables.

Your first is a Lenovo Y510p. The second is either any of Asus, MSI, or something Acer would offer.

The amount of ram won't be much of a factor once at 8GB unless doing video edit work, and even then, the CPU means more, and a hard drive speed.

Some games take better to SLI than others; generally newer games do better.
1) G3D tests for ONE GT 755M vs 850M - about 1700 vs 2000
You get about 18% improvement in one GPU systems
http://www.videocardbenchmark.net/gpu_list.php
SLI Guidance range is 50% to 90% gain, saying that in almost every game, GT 755M is better

2) GTX 850M is 17% below GT 755M SLI on average:
http://www.game-debate.com/...t-755m-sli
Assuming a normal 2GB GDDR5

3) This slow load table with a lot of data is gaming to GPU
http://www.notebookcheck.net/...849.0.html
GT 755M SLI rank 43 but the gtx 850M showing a better rank 40

So, we have some disagreement. Selecting both entries and all games and RESTRICT
Assassin s Creed IV: Black Flag MUCH better with GT 755M SLI
Call of Duty: Ghosts MUCH better with GT 755M SLI
Battlefield 4 MUCH better with GT 755M SLI
F1 2013 (2013) about even
Saints Row IV (2013) MUCH better with GT 755M SLI
Dota 2 (2013) about even
GRID 2 (2013) both good, edge to GT 755M SLI
Metro: Last Light (2013) lower settings to GTX 850M and Ultra to GT 755M SLI
BioShock Infinite (2013) MUCH better with GT 755M SLI
Tomb Raider (2013) MUCH better with GT 755M SLI
Crysis 3 (2013) the higher the settings, the better is GT 755M SLI

Continuing through, the GTX 850M is misplaced on the table when looking at actual performance and the placement was the website's expert paper judgement. Modern games are set up to handle SLI

My own thinking is old games you don't need so much power anyway, and newer games are SLI designed.

The last to consider is:
The GT 755M SLI gets hotter. GT 755M is a 50 watt GPU and so two running full are 100 watts.
The GTX 850M runs around 45 watts
You need a laptop cooling pad for extended play with the Y510p
The Lenovo Y510P has some reports of touchpad issues. That is it's main flaw. Most gamers are using USB mice anyway. The advice is if you get a funky touchpad, you could send it for replacement. It was mostly the early models. Every laptop has its flaws.

In the USA i7 quad core versions 1080p, the Y510p SLI version (at Newegg) is cheaper than Asus or MSI GTX 850M. I don't trust Acer.
http://www.rescuecom.com/...-2014.aspx

Lenovo got serious with their new Y50 at Lenovo.com using NVIDIA GeForce GTX 860M 2GB or 4GB and offers an optional 15.6'' UHD LED Glossy (3840x2160).
The GTX 860M is on a par with GT 755M SLI
GTX 860M reports 50 watts power.
Take a look at the Y50 versus the GTX 850M models if available.
They may have sealed the battery in on the Y50. Check the GTX 850M's for sealed battery. It is a trip to the shop in a few years for a new battery.

It is all about cost. There's a GT 755M SLI with an i5 at Newegg priced relatively cheap at $870