I think the only way to get a definite answer to your question is to have a chimney contractor look at your fireplace and oil furnace. My guess is that it is possible to put in a second flue IN PRINCIPLE, but it would have to be sealed so that it does not communicate in any way with the flue for the furnace. Whether there is room in your chimney to accomplish this can only be determined by someone with the right expertise/certification after a close inspection of your system. In any event, I would not let anyone talk you into a makeshift system that does not comply with your code. If you do that and you have a fire, your insurance company will not pay.

Oh, I am going to have someone from a chimney place come out and look at it, and will absolutely not do anything that's not to code!

I was just hoping to get some informed opinions beforehand so I have some background info and can think of the right questions to ask.