How do you mean? I'm connected from my Macbook Pro to two (non-Apple) external monitors (one on thunderbolt, other on HDMI) and they extend just fine - no mirro

How do you mean? I'm connected from my Macbook Pro to two (non-Apple) external monitors (one on thunderbolt, other on HDMI) and they extend just fine - no mirroring or clamshell mode. If anything, clamshell mode is the buggy one - it has a tendency to go into or come out of sleep mode when I don't want it to.