Apple Display connected to a Mac Pro and a PC

Hi

I have two monitors a Dell onr and an Apple Dispaly one. I also have a Mac Pro and a PC. Basically both screen are connected to both computers, thus in total I am using four cables which are:

1) thunderbolt 2 to thunder bolt 2, whereby one of its ends is connected to the Apple Display and the other end is connected to an adapter (thunderboot 2 to USB C) which connects back to a PCIexpress card connected to the PC.

2) Apply Display default attached cable with the other thunderbolt 2 end connected to the Mac Pro.

3) Mini DP to Display Port, one end connected to the Dell monitor and the other end connected to the PC.

4) Mini Dp to Mini DP, whereby one end is connected to the Dell monitor and the other end is connected to the Mac Pro.

On the Dell monitor I can easily select the source on it to decide whether I want to view the content of the Mac Pro or the PC when both are tuned on. However with the Apple display I cannot figure out a way to select the source when both computers are turned on. Is there a way to do this?

The other problem is, if I turn on only the PC the Apple Display will be detected and turned on. However, if I only turn on the Mac Pro the Apple Display will not be detected unless I disconnect the cable connecting the PC to the Apple Display, any idea why this is the case?