I am looking into a new heater and first time air conditioner. I live in a two story house in Pleasanton CA. The house is about 4000 sq ft (2000 on top and 2000 on bottom). The house was built in about 1910 and originally had a gravity furnace. The duct work is old and there are a couple of ducts in walls that I think leak. I can feel warm air flowing into closets. I've received bids from two contractors which were very different in approach.

Contractor #1 bid a Carrier MSA 120-20 furnace and a Carrier TXA060 air conditioner. This contractor told me that because the ducts in my house are located on the interior walls that a two speed furnace really wouldn't be very helpful. He said that two speed furnace are effective when the ducting is located on the exterior walls and the low speed can be used to keep the temperature constant. This contractor said that the only way to fix the ducts was to open the walls. He said that the spray in sealants would only help with tiny leaks and likely wouldn't help with the gaps in 90 year old ducting.

Contractor #2 bid created a couple of bids. The likely one was for two Trane XV90 furnaces (one upstairs and one downstairs). He also bid two Trane XL19i air conditioners. These are all two speed units. This contractor did not mention any concerns about location of the ducts. He recommended two separate units because Trane didn't make a single unit that could handle the 9+ tons of heating needs. He ran a heat calculation. This contractor also said the best way to fix any leaking duct would be to open the walls but did recommend using the spray in sealant first. He did express a concern that sometimes the framing of the house was used as the ducting and no ducting material was put in interior walls.

Please provide me with your impressions of the above proposals. In specific:
1. Are the advantages of a two speed furnace really for modern duct work and not for an old house with ducts from a gravity furnace?
2. For old ducting can any liners be pulled through to handle the air flow from the new heater and air conditioner? Do spray in sealants work for the gaps found in 90 year old ducts?
3. Are the products being recommended "good" solutions or are there other vendors I should be considering given my location and type of house.

Please let me know any other questions you'd recommend I ask in making my selection. Also any recommendations on requirements that I should put in the contract.