RE: Alienware 18 with dual AMD 290x connected to Asus pb287q 4k monitor goes on and off -unusable
I had similar issue to OP with older M18x R2 and dual 680M's , I did exactly as he did and custom sized and had no issues yet, I used HDMI(monster cable) but on a 65 inch Samsung 4k tv ..
anyway other issue with only 2 monitors its actually a limitation of the system, I run 1 external and the laptop or 2 externals, shutsdown laptop screen..
Alienware X51 R3
M18X R2 (again)
Inspiron 11 Flip , Studio 1535 ,Inspiron 6400
M18X R2, M17X R2 ,M17X R1, XPS M1730 & M1530, Inspiron 17R
I answer questions here, but