More information would be helpful, but here's what I think you have going.
It sounds like you're outside the U.S., probably in the UK or India where red, yellow and blue are used for hot wires and black for neutral?
What we call a 3-Way switch here in the U.S. (because it has 3 terminals on it) is called a 2-Way switch in the U.K. and areas using the same terminology, because it controls the lighting from 2 different locations.
So the 2-Way switch is the switch with 3 terminals on it, labeled L or C (common), L1 and L2, plus possibly a 4th terminal for the "earth" wire.
In the "double switch box" (it's a double-gang box with two switches), you have 1 single pole switch, and 1 "2-way" switch.
The single pole switch controls a light by itself, simple. That's connected to the cable with 3 wires (red, black, earth), which is a "switch leg." The red and black are both hot wires in this case, one bringing power to the switch, and the other taking power back to the light.
The 2-Way switch (called 3-Way switch in the U.S.) is the one connected to the cable with 4 wires (red, blue, yellow and earth). The "common" is the incoming hot wire, and both L1 and L2 are hot, depending on the position of the switch, feeding power through to another 2-Way switch (then out the common wire on that other switch).
Your "single switch" on the other end should have also been a 2-Way switch. So there should be a cable with red, blue, yellow and earth wires in that box.
The 2-Way switches work as a pair, bringing power in the common wire on one switch, and out the common wire on the other switch. The other two wires are "travelers" and carry electricity between the switches.
So... you could try this:
Leave the wires alone on the 2-Way switch that's still connected. Note which colors are connected to the common and L1 terminals.
In the "single switch box" on the other end, connect the common and L1 wires with a wirenut. Cap off the other wire with a wirenut.
Try the switch, it should switch your light on and off.