I am assuming that each of the two switches is to control the two 12 volt loads and that you have the following four conditions.
1, both loads off.
2. Load 1 on and load 2 off.
3. Load 2 on and load 1 off.
4. Load 1 and load 2 on.
Following sagor1's suggestions this could be done using two DPST switches. (But DPDT switches may be easier to obtain.) This would not require any relays. This may require more wires between the switch locations and the other parts.
Diagram 1 shows the way I would do it. (Depending on the physical location of all of the parts.)
View attachment 137147
If you have already wired it up as in your diagram then adding a diode between each 24 volt relay contact output and the input of the converter would probably be the simplest way. (The diode cathodes to the converter input.)
The diode current rating would need to handle the maximum input current to the 24 to 12 volt converter.
Probably the simplest way to mount the diodes would be to use a bridge rectifier as shown in diagram 2. The negative output of the bridge would not be connected to anything. Using the diodes would drop the input voltage to the converter by about 0.6 volts but I don't think this would matter.
Les.