Two would be best option for zoning the two floors to increase comfort level, but the added expense has always been a deterrent for many people. I personally would do two furnaces and the radiant in the basement.

Second vote for two separate systems. Zoning a single system rarely works out as most installers don't have a clue how to do this correctly (despite what they'll tell you).

I guess another option would be to put radiant heat in the basement and on the main floor, then use the forced air for air movement and heating the upper?

You'll need to weigh the additional upfront and maintenance costs for two systems versus the cost of a single, higher-capacity system with zone controls. Your HVAC contractor should be able to help you make an informed choice. Either option should be able to maintain comfortable conditions inside your home. Keep in mind that a single system with zoning will likely require more space for ductwork than systems where each is dedicated to a single level of your home. While it's always a good idea to plan for duct work in the architectural design phase to avoid an unpleasant surprise during construction, its' even more so for a single system serving multiple floors.

I have a similar size house and basement with two furnaces. The two furnaces were an upgrade option when the house was purchased by the previous owner. I am always thankful the option was chosen 40 years ago when the house was built. This is money well spent in my opinion.

Basements below ground have a small heat loss. The radiant floor heating in the basement is something you could do without if you need to balance HVAC budget if adding the second furnace. One option is to insulate the floor and add a zone off the first floor furnace. If you will be adding AC then the this added zone will help reduce the basement's humidity level.

"If you will be adding AC then the this added zone will help reduce the basement's humidity level." As you noted, Mike, basements have small heat loss. They also typically have small heat gain so they rarely call for air conditioning. If they do, air conditioning supplied by a 1st floor system would require the system to have a high turndown capability. We find that dehumidifying basements typically requires installation of a dehumidifier which is integrated into the HVAC.

Prior to installing the Carrier Infinity furnaces and AC in my house, I needed to run a dehumidifier in the basement to keep the humidity at an acceptable level. During the installation I had a return vent and one supply vent added to the finished area of the basement. The humidity level is not comfortable throughout the summer. The dehumidifier is collecting dust in the unfinished part of the basement.

I agree if the basement has a high humidity level, a dehumidifier may still be required. In my case it was not needed. The extra money spent on a communicating variable speed furnace and 2-stage AC has been offset by not running a dehumidifier.

"mike_home: The humidity level is not comfortable throughout the summer. The dehumidifier is collecting dust in the unfinished part of the basement."

Do you perhaps mean the humidity level is now comfortable throughout the summer? Else the dehumidifier wouldn't be sitting unused. :-)

Yes now comfortable. At least someone is reading my posts. 😊