Just get two identical air conditioners. You'll need two because when you take them apart, you will notice that there's only one motor for the evaporator and condenser.
Before you cut any copper lines..... use your recovery system to recover all the refrigerant from both of them. You'll first need to install a temporary recovery port, as you'll find that there is none as the units come from the factory.
Next put the unit that you plan on using as the condenser in the attic. Put the unit you plan on using as the evaporator / air handler in your closet.
Make sure that you remove the condenser fan blade from the closet unit, as you will find that it's designed to sling the condensate over the condenser coil as a means of improving efficiency, and eliminating the need to have the condensate drip out of it. Or, leave the blade on, as it provides necessary weight for the motor.
Now design something to collect the condensate. Drill a hole in the tray at the base of the closet unit. Look for a tube which drains the water from under the cold coil. Splice into this tube, and put a condensate pump below the closet AC. You should pump the condensate to the condenser, and allow the water to be slung as per the original design. We presume you have good air turnover in the attic, as you'll be pumping a bunch of humidity up there.
Now run the new line set, and hook er up. You'll want an insulated suction line. If you want really good subcooling, tape the lines together, and insulate around both of them. 1/4 and 3/8 copper should work well. You will need to come up with an oil separator, and a pump system to get the oil back up to the compressor, as it will pool in the evaporator. Call the manufacturer of the air conditioners, and ask their engineers what the recommended charge and oil volume is when you convert the system to split operation. They'll need to know your line size and length, and what method you are now using for metering. They might need to know what volume accumulator, oil separator, and dryer you used for your redesign.
You may wish to charge by gage pressures, and superheat. Which would be the best way to go. Charge by weight will be uncertain.
Definitely remove the evaporator fan blade from the AC in the attic. Fairly light weight, and not needed.
Next is the electrical portion. You will need to control the (now split) compressor, and evaporator / condenser fans, along with the oil pump. A new digital thermostat, and appropriate contactors ought to do it. You'll also need a transformer to supply power to the T-stat, and the contactors.
Pretty simple, and straight forward. Why go out and buy a split system when you can spend a bunch of time and convert a couple of window shakers for about twice the price? The adventure is what counts.