Start with evidence
Run the tests that separate the likely branches
Screen Resolution TestShow browser viewport, screen dimensions, and device pixel ratio.
Run →Monitor Refresh Rate TestEstimate display refresh timing using animation frames.
Run →Display Raw Bandwidth CalculatorEstimate raw uncompressed pixel data rate from resolution and refresh rate.
Run →USB-C Display Path Readiness CheckCheck four documented prerequisites for sending video through USB-C, Thunderbolt, a dock, cable or adapter to a display.
Run →Decision table
What the observation changes
ObservationWhat it meansNext action
Each monitor works alone but not togetherBoth panels and basic Mac outputs work; combined topology/bandwidth is higher priority.Compare the pair with Apple's supported combinations and test direct connections.
One monitor never works on either portThat display/cable path is suspect.Swap cable/input and test the known-good monitor on the same path.
Both work directly but not through dockDock is the dividing line.Check dock Thunderbolt/display capabilities and firmware.
Sequence
Work from reversible checks to escalation
- 01
Confirm M5 model.
- 02
Test each display individually and directly.
- 03
Lower resolution/refresh temporarily to test bandwidth/compatibility.
- 04
Add the dock/adapters back after direct paths are proven.
Likely areas
Keep these branches separate
Dock bandwidth/topologyCable/adapter capabilityUnsupported resolution/refresh pairDisplay input/EDID negotiation
Repair boundary
When to stop changing settings
If neither Thunderbolt port can drive a known-good supported display path, run Apple Diagnostics and contact Apple support.