Split one furnace into two?

[Mark Smith]

Hello all,

I own a two family which I pay the heat for; there is only one furnace.  The furnace is relatively old, so I've always toyed with the idea of separating the heat when that furnace kicks the bucket.  I'm wondering if anyone here has gone through this process before, and what I might be getting myself into.  Does the duct work throughout the house need to be completely redone?  Obviously there would be the expense of two furnaces plus an extra meter and some piping.  I'm wondering if the whole job can be done for under $10,000.  I pay upwards of $300/month for gas in the winter, so it's a solid hit to the cash flow.  I probably average $230/month for the whole year, so if I were to get rid of this expense I would make my money back within 4 years.  There would even be room to drop rent slightly.  The house makes good money as it is, I'm just trying to plan ahead.  

The two water tanks are also on the same gas meter as the furnace.  If I had two furnaces installed on two separate gas meters, would it be possible to link each water heater to the furnace that services the same unit?  Sorry, that might be a stupid question, I'm clearly not an HVAC pro!  What are your thoughts?

Thank you!