Anthony,
that's what you get when customers want extra's on low budget machines: problems. In my experience it's the one or the other: low budget or extra's, never both.
ramcr,
from your post I think you're looking for a circuit where two lightbulbs are connected in series between the power rails with the center connected to the PLC output. Unfortunately, most PLC's have open collector outputs, so you can't use this solution.
I have seen this circuit a lot in the old days when we were using discrete logic (eg. 7400 family) where some functions had totempole outputs. In that case this is indeed a simple way to indicate both the ON and the OFF-states.
Kind regards,