Sounds like more of a trick question than anything else. Two 24DC supplies, but one of them has + as the common. That's the only way you could get 48DC on the input.
PLC1 has relay output and soucing the -. +24 is wired to common return.
PLC2 has sinking inputs. -24DC is wired to common return (shared between the 2 supplies), and +24DC is wired to input common.
Does anybody tie +24 to ground? I don't think so.