This was not a test. This circuit works. Those are flashers, they get power from 12 v battery and the out terminal the "load" terminal goes to each switch. The DPST has just the off or on position to turn on both led's at the same time and the flasher, flashes them at the same time.
"Could the DPDT switch be replaced with a SPST switch?"
It's a DPST switch it feeds both led's with 1 throw.
Thanks for liking my schematic.