It's perfectly okay to connect a second GPU, but the same problem could very well occur on that one as well. That's why it's best to first determine what might be causing the problem.
Here's a simple experiment:
- Disconnect the two monitors from your 5700 XT
- Connect only the TV
- Does the GPU still go to 100%?
- If so, then adding a second GPU (RX 580) may not be a solution.
Any available long slot is okay. It's best to use the first and second slots, in which case both operate at x8. This is more than 100000% okay because you're not playing games or using the GPU for extreme tasks. Fortunately no changes to
config.plist will be needed. MacOS will auto-detect both GPUs without any issue.