Just put the heat pump coil on top of the gas furnace and disconnect the electric heat elements.

You are making this way more complicated than it needs to be.

Thanks for your reply. Yes, I thought about doing that but it would not be easy. I would have to have a box custom built to contain the A-coil on top of the new coleman furnace. Also the refrig. lines would have to be changed to a different height and lastly the control board for the heat pump is built into the present air handler.As it is now I just have to hook up the return air duct and the supply airduct to the new furnace. Would the blower fans in each furnace be enough to keep the supply air from looping back through the unit not being used. Thanks

fabricating a plenum to handle the evaporator coil is normal prodeure in the hvac industry. And to suggest haveing 2 pieces of equipment on the same single duct system is not gonna fly. Do as brickeyee suggested

Thanks guys for the replys. I am going to do it the way you suggested. I called a sheet metal fab shop today. I will be taking all the measurments to him. I will mount the control board for the heat pump close to the control board for the furnace. The only thing they will have in common will be they use the same blower fan. Thanks

Not sure why you think you need to control them off two different controls. A heat pump is commonly used with a furnace in what is known as a dual fuel setup. The heat pump provides most of the heat, and the furnace is used when the heat pump can no longer keep up. You can use a dual fuel kit to control this setup, or a dual fuel thermostat. I would suggest having a professional do the work unless you're in the field.

The money spent having a professional set this up will be regained in lower bills later I am sure. I have no doubt you can set it up and make it work, but you want the end result to leave as much money in your wallet as the months go by.

Of course you can have 2 units tied into 1 duct system. As to your actual question, i'll answer it. You will need to either manually or mechanically block off the unit not being used...Fof instance, install the gas furnace facing the existing heat pump etc, then slide a rigid baffle in front of the heat pump, blocking any pass through air flow when using the gas furnace...lol, simple as moving one baffle incorporated into the supply plenum. 2 separate T stats, separate power supplies, same existing duct system, supply and return. I never did like the way 98% of people ask you more questions than you ask them.....Why, Why, Why.....dunno "Why" they need to know Why, just answer the question OP posted, lol.