Digital circuit to sense both high side and low side switches
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Opto-isolator to detect current flow in LED circuit.

The problem with your circuit is that with S1 and S2 open both ends of LED2 are floating and your GPIO input will be too so the logic level will be undefined.

A simple solution is to monitor the current in the LED circuit. This can be achieved with an opto-isolator as shown in Figure 1.