Linux, U2520D, U2515H, daisy chain, issue with DDC/CI

Hi,

I have two monitors:
- main: U2520D (DP 1.4 into NVidia GeForce 1070 Ti)
- chained: U2515H (DP 1.2 into U2520D DP-out)

I want to use DDC/CI to control my brightness on both monitors. However, so far I can only do it on the main monitor. This is to mean that if I set the brightness it only changes on the main monitor which is directly connected to the GPU and the monitor that is daisy chained remains unaffected. I've tested them individually and if each one is directly connected to the GPU, then setting the brightness works. But it doesn't when daisy chaining. Does anyone know if this is possible? And if so, how? If not, what are my alternatives - I would like to minimize the number of cables going to the PC.