1.4V instead of 0V when Arduino signals a LOW
You've got two outputs driving the same line. If one pulls high, and one pulls low, they short each other out.

The internal resistance of both drivers will result in a mid-level voltage on the signal line. Typically closer to 0V than VDD as NMOS transistors (used to pull down) are typically stronger than PMOS (used to pull up).

How you resolve it depends on how your opto-coupler is driven (hence we always ask for a proper schematic). If you connect it from VDD (anode) to signal (cathode), then you just need to make both outputs open drain (i.e. they never pull high, only low or floating). Alternatively use a AND gate which will drive the opto-coupler high only when both inputs are high.