Got 2 gang light switch, common load, design circuit to know ON/OFF state of switches on each wire
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. An unusual approach.

I offer this solution more as a puzzle answer than as a solution that I would recommend. How it works:


Almost certainly the 48 V supply you are using for your LEDs is an SMPS (switched-mode power supply) has a full-wave rectifier on the input so it can be run on full-wave rectified mains. While the current through the internal bridge will be the same only two of the four diodes will be employed so there is a risk that they may be stressed.
Zero-cross detection circuits provide a simple and opto-isolated means of monitoring the switches' on/off status. You're not interested in the zero-cross but rather that the mains turns on. There are plenty of examples on this and other sites but bear in mind that a peak detector will suffice for you so you may be able to design for minimum power dissipation in the zero-cross circuits.
BR1 and BR2 rectify the AC coming from SW1 and SW2 and prevent backfeed from one circuit to the other.


It gets around the relay back-feed problems and avoids current-sensing solutions which might not work if the current isn't shared equally when SW1 and SW2 are both on.