Why does each monitor work alone, but a second display is not detected?

Check the Mac's supported multi-display configuration and the complete connection path. A monitor working by itself does not establish that both monitors can run together at the same resolutions and refresh rates.

Check the exact chip name in About This Mac. For example, a MacBook Pro with M3 and one with M3 Pro have different display limits. M3 requires the lid closed for two native external displays. M3 Pro supports two within its specified combinations. Apple's MacBook Pro display-support guide lists the resolution, refresh-rate and port combinations for each chip.

Where the Mac requires a closed lid for a second native external display, soft-disconnecting the built-in screen in BetterDisplay is not a substitute. The built-in display can retain its resources while soft-disconnected. That action does not guarantee an extra native external-display connection with the lid open.

Then compare a simpler setup:

  1. Test each display through a direct connection to the Mac, bypassing a dock where possible. Check the cable and adapter's video capabilities, not just the connector shape.
  2. Try lower supported refresh rates and reconnect both displays. High-refresh operation that works with one monitor may not fit a two-monitor arrangement.
  3. Disconnect and reconnect the display cables and power. Note whether connecting the monitors in a different order changes detection.
  4. Check macOS's accessory-connection prompt, if shown, and follow Apple's external-display troubleshooting steps.

After switching a KVM back to the Mac, try Tools > Redetect Displays to request detection and refresh BetterDisplay’s configuration. If macOS knows about a display that is disconnected through the app, Tools > Connect All Displays may bring it online. These actions cannot recover a signal path that macOS does not detect at all. Connect All Displays can also reconnect the built-in display and suspend its automatic-disconnect rule, so check the resulting arrangement.

An adapter change can alter the result, but no particular adapter guarantees compatibility with every pair of monitors. Likewise, a docking station does not itself increase the Mac's native display limit. BetterDisplay cannot force macOS to use an unsupported native display arrangement.

If both displays appear but the desired HiDPI or refresh-rate combination is missing, see HiDPI refresh-rate limits. Record the exact Mac chip, monitor models, ports, adapters and working one-display modes when investigating a remaining detection problem.

For the rendering background, see how macOS scaling, HiDPI and LoDPI work.