I'm considering replacing my furnace (gas and radiators) with a forced air system for both heating a cooling. Currently the air conditioning does not service the whole house.

I'm trying to understand the configuration of the system inside the house. The location requires a vertical loop, drilling etc so that section is pretty clear. Most of the discussions seem to be around the different loop parts of the system. Back to my system. It will be a two zone system, one servicing finished basement and main floor (2000 sq ft) and the other zone for 2nd floor and finished attic (1500 sq ft). I also want to have a hot water component.

Now the question; can I go with one "split" connected to the outside loop and then connect two air handlers (one in basement and one in attic) for heating and cooling and the water heater to the split? Note that I will be running two thermostats, one for each zone. Also can the air handlers be of different size in this configuration? Or am I missing some component here?

It sounds like your options are two separate small systems or a single system with two zones.

Zoning is accomplished using one air handler and two (or more) electromechanical flaps inside the ductwork which send supply air only where it is needed. A single outdoor loop field can serve either arrangement. Zoning should be slightly cheaper up front (although quipment savings might be eaten up by extra ductwork and zone board) Zoning might be quieter for whatever floor the airhandler does not occupy.

Two small systems would be simpler for most contractors to understand and install and offers redundancy. A single zoned system might be more favorable and simpler for the hot water recovery, but that's not a major issue.

Some minisplit ductless systems can run up to four blower units off a single compressor, but to my knowledge that technique hasn't made it to geothermal

Thanks for the reply.

OK, so there is really not  a single "split" working with two air handlers option? Unfortunately I don´t think I have the option of ducting between basement and attic.

A vertical chase in a closet could work wonders.

Or not - go with two units.

Just make sure your zone and room load calcs are done right

Daikin or Mitsu ductless minisplits might be a viable option if your house is fairly open floor plan. Those are the ones that drive multiple evaporators from a single compressor.