Apple's specs say you can plug the LG 5K into the iMac and they should both be fine at 5K. Has anyone tried adding a 1440p on top of this? e.g. via the other TB3 port (say, into a TB dock that outputs DisplayPort)? EDIT: So I did some quick research (prob not very accurate). Assuming billions of colours means 10-bits per pixel, and I apply a colour depth factor of 1.2 and coefficient of 10 Then (according to Apple): One 5120‑by‑2880 (5K) external display at 60Hz with support for 1 billion colours per my calc would be: 5120 x 2880 × 60 × 1.2 × 10 10.62 Gbps per channel 31.85 Gbps total (RGB) This is within the 40Gbps TB3 limit. Checking the maths again: Two 3840-by-2160 (4K UHD) external displays at 60Hz with support for 1 billion colours per my calc would be: 3840 × 2160 × 60 × 1.2 × 10 (x2) 11.94 Gbps per channel 35.83 Gbps total (RGB) This is within the 40Gbps TB3 limit. So what I was proposing would be ((5120 x 2880 × 60) + (2560 × 1440 × 60)) × 1.2 × 10 13.27 Gbps per channel 39.81 Gbps total (RGB) Very close....but probably over the 40Gbps of TB3 given the maths isn't at all accurate.