Fireplaces, Heating Stoves, Flues and Chimneys - Extending Dual Chimney

I'm adding a second floor to my home and need to extend the chimney. My oil burner exhausts into chimney and I want to be able to use the fireplace to burn wood. I got an estimate of $4,000 to 5,000 to extend the masonry chimney with a new liner. I want to do it myself. I will have a 26" space to work with between the first and second floor. I want to know if I can do the following:
1. Remove the portion of the existing chimney that extends above the first floor ceiling.
2. Install new Flue liner in existing chimney.
3. Use "Y" to join the two vents into one (within the 26" space mentioned above).
4. Use insulated vent pipe to continue the vent through the new second floor and roof.
5. I could then attach a vent pipe enclosure that looks like a masonry chimney.

Well, that's the plan. The reason for this is that I am reluctant to attempt the brickwork myself, and I'd like to reduce the amount of space taken up by the chimney as it passes through the second floor.

Any suggestions? Is this doable? With modifications?

Thank you for any advice.

Check in with your local building inspector regarding any changes to your structure and to your chimney. Codes tend to vary from area to area.